Fair Value Measurements - Gross Components of Purchases, Sales, Issuances and Settlements (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Assets:
Purchases	$ 419,000,000	$ 15,000,000	$ 582,000,000	$ 97,000,000
Sales	(48,000,000)	(1,000,000)	(100,000,000)	(7,000,000)
Issuances and Settlements	(107,000,000)	(216,000,000)	(344,000,000)	(530,000,000)
Purchases, Sales, Issuances and Settlements, Net	264,000,000	(202,000,000)	138,000,000	(440,000,000)
Liabilities:
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	(120,000,000)	108,000,000	(112,000,000)	346,000,000
Purchases, Sales, Issuances and Settlements, Net	(120,000,000)	108,000,000	(112,000,000)	346,000,000
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	9,000,000	(24,000,000)	11,000,000	(27,000,000)
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	1,000,000	1,000,000	6,000,000	1,000,000
Derivative liabilities, net
Liabilities:
Purchases		0		0
Sales		0		0
Issuances and Settlements		156,000,000		374,000,000
Purchases, Sales, Issuances and Settlements, Net		156,000,000		374,000,000
Fortitude Re funds withheld payable
Liabilities:
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	(120,000,000)	(27,000,000)	(112,000,000)	(7,000,000)
Purchases, Sales, Issuances and Settlements, Net	(120,000,000)	(27,000,000)	(112,000,000)	(7,000,000)
Other Liabilities
Liabilities:
Purchases		0		0
Sales		0		0
Issuances and Settlements		(21,000,000)		(21,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	(21,000,000)	0	(21,000,000)
Bonds available for sale
Assets:
Purchases	396,000,000	15,000,000	545,000,000	82,000,000
Sales	(35,000,000)	0	(83,000,000)	(6,000,000)
Issuances and Settlements	(99,000,000)	(177,000,000)	(307,000,000)	(369,000,000)
Purchases, Sales, Issuances and Settlements, Net	262,000,000	(162,000,000)	155,000,000	(293,000,000)
Bonds available for sale | Obligations of states, municipalities and political subdivisions
Assets:
Purchases				1,000,000
Sales				0
Issuances and Settlements				0
Purchases, Sales, Issuances and Settlements, Net	0	0	0	1,000,000
Bonds available for sale | Non-U.S. governments
Assets:
Purchases	0	4,000,000	0	4,000,000
Sales	0	0	0	0
Issuances and Settlements	(1,000,000)	(4,000,000)	(1,000,000)	(4,000,000)
Purchases, Sales, Issuances and Settlements, Net	(1,000,000)	0	(1,000,000)	0
Bonds available for sale | Corporate debt
Assets:
Purchases	2,000,000	5,000,000	5,000,000	11,000,000
Sales	(2,000,000)	0	(6,000,000)	(3,000,000)
Issuances and Settlements	(26,000,000)	(27,000,000)	(150,000,000)	(68,000,000)
Purchases, Sales, Issuances and Settlements, Net	(26,000,000)	(22,000,000)	(151,000,000)	(60,000,000)
Bonds available for sale | RMBS
Assets:
Purchases	8,000,000	0	8,000,000	0
Sales	0	0	(3,000,000)	(1,000,000)
Issuances and Settlements	(47,000,000)	(75,000,000)	(101,000,000)	(149,000,000)
Purchases, Sales, Issuances and Settlements, Net	(39,000,000)	(75,000,000)	(96,000,000)	(150,000,000)
Bonds available for sale | CMBS
Assets:
Purchases	0	0	0	0
Sales	0	0	(4,000,000)	0
Issuances and Settlements	(1,000,000)	(18,000,000)	(1,000,000)	(18,000,000)
Purchases, Sales, Issuances and Settlements, Net	(1,000,000)	(18,000,000)	(5,000,000)	(18,000,000)
Bonds available for sale | CLO/ABS
Assets:
Purchases	386,000,000	6,000,000	532,000,000	66,000,000
Sales	(33,000,000)	0	(70,000,000)	(2,000,000)
Issuances and Settlements	(24,000,000)	(53,000,000)	(54,000,000)	(130,000,000)
Purchases, Sales, Issuances and Settlements, Net	329,000,000	(47,000,000)	408,000,000	(66,000,000)
Other bond securities
Assets:
Purchases	1,000,000	0	1,000,000	14,000,000
Sales	0	(1,000,000)	0	(1,000,000)
Issuances and Settlements	(2,000,000)	(4,000,000)	(7,000,000)	(8,000,000)
Purchases, Sales, Issuances and Settlements, Net	(1,000,000)	(5,000,000)	(6,000,000)	5,000,000
Other bond securities | Corporate debt
Assets:
Purchases, Sales, Issuances and Settlements, Net	0	0	0	0
Other bond securities | RMBS
Assets:
Purchases	0	0	0	3,000,000
Sales	0	(1,000,000)	0	(1,000,000)
Issuances and Settlements	(1,000,000)	(1,000,000)	(2,000,000)	(2,000,000)
Purchases, Sales, Issuances and Settlements, Net	(1,000,000)	(2,000,000)	(2,000,000)	0
Other bond securities | CLO/ABS
Assets:
Purchases	1,000,000	0	1,000,000	11,000,000
Sales	0	0	0	0
Issuances and Settlements	(1,000,000)	(3,000,000)	(5,000,000)	(6,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	(3,000,000)	(4,000,000)	5,000,000
Equity securities
Assets:
Purchases	21,000,000		35,000,000
Sales	(13,000,000)		(17,000,000)
Issuances and Settlements	0		0
Purchases, Sales, Issuances and Settlements, Net	8,000,000	0	18,000,000	0
Other invested assets
Assets:
Purchases	1,000,000	0	1,000,000	1,000,000
Sales	0	0	0	0
Issuances and Settlements	(6,000,000)	(36,000,000)	(30,000,000)	(40,000,000)
Purchases, Sales, Issuances and Settlements, Net	(5,000,000)	(36,000,000)	(29,000,000)	(39,000,000)
Other assets
Assets:
Purchases		0		0
Sales		0		0
Issuances and Settlements		1,000,000		(113,000,000)
Purchases, Sales, Issuances and Settlements, Net	$ 0	$ 1,000,000	$ 0	$ (113,000,000)