Fair Value Measurements - Changes in Level 3 Recurring Fair Value Measurements, Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Assets:
Fair Value Beginning of Period	$ 3,898	$ 3,481
Net Realized and Unrealized Gains (Losses) Included in Income	6	4
Other Comprehensive Income (Loss)	(40)	44
Purchases, Sales, Issuances and Settlements, Net	(104)	(126)
Gross Transfers In	14	47
Gross Transfers Out	(40)	(317)
Other	(130)
Fair Value End of Period	3,604	3,133
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	5	4
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	$ 33	$ 27
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Revenues	Revenues
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Other comprehensive (loss) income	Other comprehensive (loss) income
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Realized Investment Gains (Losses)	Realized Investment Gains (Losses)
Bonds available for sale
Assets:
Fair Value Beginning of Period	$ 3,451	$ 3,010
Net Realized and Unrealized Gains (Losses) Included in Income	8	(1)
Other Comprehensive Income (Loss)	(40)	44
Purchases, Sales, Issuances and Settlements, Net	(103)	(107)
Gross Transfers In	14	7
Gross Transfers Out	(40)	(231)
Other	0
Fair Value End of Period	3,290	2,722
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	33	27
Bonds available for sale | Obligations of states, municipalities and political subdivisions
Assets:
Fair Value Beginning of Period	4	3
Net Realized and Unrealized Gains (Losses) Included in Income	0	0
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	0	0
Gross Transfers In	0	0
Gross Transfers Out	0	0
Other	0
Fair Value End of Period	4	3
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Bonds available for sale | Non-U.S. governments
Assets:
Fair Value Beginning of Period	23	7
Net Realized and Unrealized Gains (Losses) Included in Income	1	0
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	(3)	0
Gross Transfers In	0	0
Gross Transfers Out	(14)	0
Other	0
Fair Value End of Period	7	7
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Bonds available for sale | Corporate debt
Assets:
Fair Value Beginning of Period	113	240
Net Realized and Unrealized Gains (Losses) Included in Income	0	(9)
Other Comprehensive Income (Loss)	(2)	10
Purchases, Sales, Issuances and Settlements, Net	(1)	(125)
Gross Transfers In	1	0
Gross Transfers Out	(26)	(1)
Other	0
Fair Value End of Period	85	115
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	6	9
Bonds available for sale | RMBS
Assets:
Fair Value Beginning of Period	1,546	1,894
Net Realized and Unrealized Gains (Losses) Included in Income	6	7
Other Comprehensive Income (Loss)	(37)	33
Purchases, Sales, Issuances and Settlements, Net	(37)	(57)
Gross Transfers In	8	3
Gross Transfers Out	0	(224)
Other	0
Fair Value End of Period	1,486	1,656
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	17	17
Bonds available for sale | CMBS
Assets:
Fair Value Beginning of Period	24	26
Net Realized and Unrealized Gains (Losses) Included in Income	0	0
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	(4)	(4)
Gross Transfers In	5	4
Gross Transfers Out	0	0
Other	0
Fair Value End of Period	25	26
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Bonds available for sale | CLO/ABS
Assets:
Fair Value Beginning of Period	1,741	840
Net Realized and Unrealized Gains (Losses) Included in Income	1	1
Other Comprehensive Income (Loss)	(1)	1
Purchases, Sales, Issuances and Settlements, Net	(58)	79
Gross Transfers In	0	0
Gross Transfers Out	0	(6)
Other	0
Fair Value End of Period	1,683	915
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	10	1
Other bond securities
Assets:
Fair Value Beginning of Period	170	164
Net Realized and Unrealized Gains (Losses) Included in Income	0	4
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	(3)	(5)
Gross Transfers In	0	31
Gross Transfers Out	0	(23)
Other	0
Fair Value End of Period	167	171
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	4	4
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Other bond securities | Corporate debt
Assets:
Fair Value Beginning of Period		1
Net Realized and Unrealized Gains (Losses) Included in Income		0
Other Comprehensive Income (Loss)		0
Purchases, Sales, Issuances and Settlements, Net		0
Gross Transfers In		0
Gross Transfers Out		0
Fair Value End of Period		1
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period		0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period		0
Other bond securities | RMBS
Assets:
Fair Value Beginning of Period	51	50
Net Realized and Unrealized Gains (Losses) Included in Income	0	1
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	(1)	(1)
Gross Transfers In	0	0
Gross Transfers Out	0	0
Other	0
Fair Value End of Period	50	50
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	1	1
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Other bond securities | CLO/ABS
Assets:
Fair Value Beginning of Period	119	113
Net Realized and Unrealized Gains (Losses) Included in Income	0	3
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	(2)	(4)
Gross Transfers In	0	31
Gross Transfers Out	0	(23)
Other	0
Fair Value End of Period	117	120
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	3	3
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Equity securities
Assets:
Fair Value Beginning of Period	55	15
Net Realized and Unrealized Gains (Losses) Included in Income	(3)	1
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	2	10
Gross Transfers In	0	9
Gross Transfers Out	0	0
Other	0
Fair Value End of Period	54	35
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Other invested assets
Assets:
Fair Value Beginning of Period	92	163
Net Realized and Unrealized Gains (Losses) Included in Income	1	0
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	0	(24)
Gross Transfers In	0	0
Gross Transfers Out	0	(63)
Other	0
Fair Value End of Period	93	76
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	1	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Other assets
Assets:
Fair Value Beginning of Period	130	129
Net Realized and Unrealized Gains (Losses) Included in Income	0	0
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	0	0
Gross Transfers In	0	0
Gross Transfers Out	0	0
Other	(130)
Fair Value End of Period	0	129
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	$ 0	$ 0