Fair Value Measurements - Gross Components of Purchases, Sales, Issuances and Settlements (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Assets:
Purchases	$ 15,000,000	$ 142,000,000	$ 97,000,000	$ 327,000,000
Sales	(1,000,000)	(149,000,000)	(7,000,000)	(179,000,000)
Issuances and Settlements	(216,000,000)	(246,000,000)	(530,000,000)	(588,000,000)
Purchases, Sales, Issuances and Settlements, Net	(202,000,000)	(253,000,000)	(440,000,000)	(440,000,000)
Liabilities:
Purchases	0	(105,000,000)	0	(316,000,000)
Sales	0	1,000,000	0	4,000,000
Issuances and Settlements	108,000,000	(22,000,000)	346,000,000	(167,000,000)
Purchases, Sales, Issuances and Settlements, Net	108,000,000	(126,000,000)	346,000,000	(479,000,000)
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	(24,000,000)	0	(27,000,000)	1,000,000
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	(9,000,000)	1,000,000	(9,000,000)
Derivative liabilities, net
Liabilities:
Purchases	0	(105,000,000)	0	(316,000,000)
Sales	0	1,000,000	0	4,000,000
Issuances and Settlements	156,000,000	22,000,000	374,000,000	143,000,000
Purchases, Sales, Issuances and Settlements, Net	156,000,000	(82,000,000)	374,000,000	(169,000,000)
Fortitude Re funds withheld payable
Liabilities:
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	(27,000,000)	(44,000,000)	(7,000,000)	(310,000,000)
Purchases, Sales, Issuances and Settlements, Net	(27,000,000)	(44,000,000)	(7,000,000)	(310,000,000)
Other Liabilities
Liabilities:
Purchases	0		0
Sales	0		0
Issuances and Settlements	(21,000,000)		(21,000,000)
Purchases, Sales, Issuances and Settlements, Net	(21,000,000)	0	(21,000,000)	0
Bonds available for sale
Assets:
Purchases	15,000,000	123,000,000	82,000,000	287,000,000
Sales	0	(153,000,000)	(6,000,000)	(179,000,000)
Issuances and Settlements	(177,000,000)	(238,000,000)	(369,000,000)	(562,000,000)
Purchases, Sales, Issuances and Settlements, Net	(162,000,000)	(268,000,000)	(293,000,000)	(454,000,000)
Bonds available for sale | Obligations of states, municipalities and political subdivisions
Assets:
Purchases			1,000,000	1,000,000
Sales			0	(4,000,000)
Issuances and Settlements			0	(2,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	0	1,000,000	(5,000,000)
Bonds available for sale | Non-U.S. governments
Assets:
Purchases	4,000,000	0	4,000,000	0
Sales	0	0	0	0
Issuances and Settlements	(4,000,000)	(2,000,000)	(4,000,000)	(2,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	(2,000,000)	0	(2,000,000)
Bonds available for sale | Corporate debt
Assets:
Purchases	5,000,000	0	11,000,000	8,000,000
Sales	0	0	(3,000,000)	0
Issuances and Settlements	(27,000,000)	(131,000,000)	(68,000,000)	(355,000,000)
Purchases, Sales, Issuances and Settlements, Net	(22,000,000)	(131,000,000)	(60,000,000)	(347,000,000)
Bonds available for sale | RMBS
Assets:
Purchases	0	46,000,000	0	170,000,000
Sales	0	0	(1,000,000)	(19,000,000)
Issuances and Settlements	(75,000,000)	(82,000,000)	(149,000,000)	(169,000,000)
Purchases, Sales, Issuances and Settlements, Net	(75,000,000)	(36,000,000)	(150,000,000)	(18,000,000)
Bonds available for sale | CMBS
Assets:
Purchases	0	0	0	1,000,000
Sales	0	(5,000,000)	0	(5,000,000)
Issuances and Settlements	(18,000,000)	5,000,000	(18,000,000)	5,000,000
Purchases, Sales, Issuances and Settlements, Net	(18,000,000)	0	(18,000,000)	1,000,000
Bonds available for sale | CLO/ABS
Assets:
Purchases	6,000,000	77,000,000	66,000,000	107,000,000
Sales	0	(148,000,000)	(2,000,000)	(151,000,000)
Issuances and Settlements	(53,000,000)	(28,000,000)	(130,000,000)	(39,000,000)
Purchases, Sales, Issuances and Settlements, Net	(47,000,000)	(99,000,000)	(66,000,000)	(83,000,000)
Other bond securities
Assets:
Purchases	0	20,000,000	14,000,000	34,000,000
Sales	(1,000,000)	4,000,000	(1,000,000)	0
Issuances and Settlements	(4,000,000)	(1,000,000)	(8,000,000)	(18,000,000)
Purchases, Sales, Issuances and Settlements, Net	(5,000,000)	23,000,000	5,000,000	16,000,000
Other bond securities | Corporate debt
Assets:
Purchases		20,000,000		20,000,000
Sales		0		0
Issuances and Settlements		24,000,000		24,000,000
Purchases, Sales, Issuances and Settlements, Net	0	44,000,000	0	44,000,000
Other bond securities | RMBS
Assets:
Purchases	0	0	3,000,000	0
Sales	(1,000,000)	0	(1,000,000)	0
Issuances and Settlements	(1,000,000)	(2,000,000)	(2,000,000)	(15,000,000)
Purchases, Sales, Issuances and Settlements, Net	(2,000,000)	(2,000,000)	0	(15,000,000)
Other bond securities | CLO/ABS
Assets:
Purchases	0	0	11,000,000	14,000,000
Sales	0	4,000,000	0	0
Issuances and Settlements	(3,000,000)	(23,000,000)	(6,000,000)	(27,000,000)
Purchases, Sales, Issuances and Settlements, Net	(3,000,000)	(19,000,000)	5,000,000	(13,000,000)
Equity securities
Assets:
Purchases				5,000,000
Sales				0
Issuances and Settlements				(1,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	0	0	4,000,000
Other invested assets
Assets:
Purchases	0	(1,000,000)	1,000,000	1,000,000
Sales	0	0	0	0
Issuances and Settlements	(36,000,000)	(8,000,000)	(40,000,000)	(11,000,000)
Purchases, Sales, Issuances and Settlements, Net	(36,000,000)	(9,000,000)	(39,000,000)	(10,000,000)
Other assets
Assets:
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	1,000,000	1,000,000	(113,000,000)	4,000,000
Purchases, Sales, Issuances and Settlements, Net	$ 1,000,000	$ 1,000,000	$ (113,000,000)	$ 4,000,000