Fair Value Measurements - Gross Components of Purchases, Sales, Issuances and Settlements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Assets:
Purchases	$ 1,597,000,000	$ 609,000,000
Sales	(184,000,000)	(781,000,000)
Issuances and Settlements	(654,000,000)	(754,000,000)
Purchases, Sales, Issuances and Settlements, Net	759,000,000	(926,000,000)
Liabilities:
Purchases	0	0
Sales	0	0
Issuances and Settlements	(130,000,000)	302,000,000
Purchases, Sales, Issuances and Settlements, Net	(130,000,000)	302,000,000
Issuances, assets	0	0
Issuances, liabilities	0	0
Transfers into Level 3 at end of reporting period, net gains (losses) not included in realized and unrealized gains and losses related to Level 3 for the period	14,000,000	(35,000,000)
Transfers out Level 3 at end of reporting period, net gains (losses) included in realized and unrealized gains and losses related to Level 3 for the period	6,000,000	(13,000,000)
Derivative liabilities, net
Liabilities:
Purchases		0
Sales		0
Issuances and Settlements		377,000,000
Purchases, Sales, Issuances and Settlements, Net		377,000,000
Fortitude Re funds withheld payable
Liabilities:
Purchases	0	0
Sales	0	0
Issuances and Settlements	(130,000,000)	(55,000,000)
Purchases, Sales, Issuances and Settlements, Net	(130,000,000)	(55,000,000)
Other Liabilities
Liabilities:
Purchases		0
Sales		0
Issuances and Settlements		(20,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	(20,000,000)
Bonds available for sale
Assets:
Purchases	1,507,000,000	584,000,000
Sales	(127,000,000)	(778,000,000)
Issuances and Settlements	(608,000,000)	(588,000,000)
Purchases, Sales, Issuances and Settlements, Net	772,000,000	(782,000,000)
Bonds available for sale | Obligations of states, municipalities and political subdivisions
Assets:
Purchases	1,000,000	1,000,000
Sales	0	0
Issuances and Settlements	0	(1,000,000)
Purchases, Sales, Issuances and Settlements, Net	1,000,000	0
Bonds available for sale | Non-U.S. governments
Assets:
Purchases	1,000,000	4,000,000
Sales	0	0
Issuances and Settlements	(1,000,000)	(4,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	0
Bonds available for sale | Corporate debt
Assets:
Purchases	6,000,000	43,000,000
Sales	(22,000,000)	(29,000,000)
Issuances and Settlements	(153,000,000)	(85,000,000)
Purchases, Sales, Issuances and Settlements, Net	(169,000,000)	(71,000,000)
Bonds available for sale | RMBS
Assets:
Purchases	55,000,000	89,000,000
Sales	(3,000,000)	(53,000,000)
Issuances and Settlements	(193,000,000)	(274,000,000)
Purchases, Sales, Issuances and Settlements, Net	(141,000,000)	(238,000,000)
Bonds available for sale | CMBS
Assets:
Purchases	0	0
Sales	(4,000,000)	(15,000,000)
Issuances and Settlements	(5,000,000)	(17,000,000)
Purchases, Sales, Issuances and Settlements, Net	(9,000,000)	(32,000,000)
Bonds available for sale | CLO/ABS
Assets:
Purchases	1,444,000,000	447,000,000
Sales	(98,000,000)	(681,000,000)
Issuances and Settlements	(256,000,000)	(207,000,000)
Purchases, Sales, Issuances and Settlements, Net	1,090,000,000	(441,000,000)
Other bond securities
Assets:
Purchases	4,000,000	16,000,000
Sales	0	(1,000,000)
Issuances and Settlements	(11,000,000)	(14,000,000)
Purchases, Sales, Issuances and Settlements, Net	(7,000,000)	1,000,000
Other bond securities | Corporate debt
Assets:
Purchases, Sales, Issuances and Settlements, Net	0	0
Other bond securities | RMBS
Assets:
Purchases	3,000,000	3,000,000
Sales	0	(1,000,000)
Issuances and Settlements	(3,000,000)	(5,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	(3,000,000)
Other bond securities | CLO/ABS
Assets:
Purchases	1,000,000	13,000,000
Sales	0	0
Issuances and Settlements	(8,000,000)	(9,000,000)
Purchases, Sales, Issuances and Settlements, Net	(7,000,000)	4,000,000
Equity securities
Assets:
Purchases	75,000,000	6,000,000
Sales	(57,000,000)	(2,000,000)
Issuances and Settlements	0	0
Purchases, Sales, Issuances and Settlements, Net	18,000,000	4,000,000
Other invested assets
Assets:
Purchases	11,000,000	3,000,000
Sales	0	0
Issuances and Settlements	(36,000,000)	(38,000,000)
Purchases, Sales, Issuances and Settlements, Net	(25,000,000)	(35,000,000)
Other assets
Assets:
Purchases	0	0
Sales	0	0
Issuances and Settlements	1,000,000	(114,000,000)
Purchases, Sales, Issuances and Settlements, Net	$ 1,000,000	$ (114,000,000)