Subclassifications Of Assets, Liabilities And Equities [Abstract]
| Concept | As at 2024-12-31 | As at 2023-12-31 | As at 2022-12-31 | 
|---|---|---|---|
| Subclassifications of assets, liabilities and equities [abstract] | |||
| Property, plant and equipment [abstract] | |||
| Property, plant and equipment | 
                                                676,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                732,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Intangible assets and goodwill [abstract] | |||
| Intangible assets and goodwill | 
                                                592,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                649,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Investment property [abstract] | |||
| Investment property | 
                                                3,364,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                3,051,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Investments accounted for using equity method [abstract] | |||
| Investments accounted for using equity method | 
                                                457,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                426,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Cash and cash equivalents [abstract] | |||
| Cash and cash equivalents | 
                                                4,194,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                7,910,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                2,246,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
| Miscellaneous assets [abstract] | |||
| Derivative financial assets | 
                                                11,767,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                12,907,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Investments other than investments accounted for using equity method | 
                                                80,593,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                90,006,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Investments for risk of policyholders | 
                                                33,025,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                30,352,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Other assets | 
                                                3,342,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                3,598,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Borrowings [abstract] | |||
| Borrowings | 
                                                3,135,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                5,451,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Borrowings, by type [abstract] | |||
| Miscellaneous liabilities [abstract] | |||
| Net defined benefit liability | 
                                                5,037,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                5,218,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Derivative financial liabilities | 
                                                8,666,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                10,132,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Deposit liabilities | — | — | — | 
| Deposits from banks | 
                                                5,550,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                5,445,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Deposits from customers [abstract] | |||
| Deposits from customers | 
                                                -
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                11,967,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Subordinated liabilities [abstract] | |||
| Subordinated liabilities | 
                                                2,007,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                2,005,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Other liabilities | 
                                                1,322,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                1,376,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Issued capital [abstract] | |||
| Issued capital | 
                                                34,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                34,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Retained earnings [abstract] | |||
| Retained earnings | 
                                                4,528,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                4,147,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Accumulated other comprehensive income [abstract] | |||
| Reserve of remeasurements of defined benefit plans | 
                                                175,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                288,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Net assets (liabilities) [abstract] | |||
| Assets | 
                                                138,595,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                150,768,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Liabilities | 
                                                128,762,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | 
                                                141,391,000,000
                                                
                                                    
                                                        EUR
                                                    
                                                
                                             | — | 
| Assets less current liabilities [abstract] |