Fair Value Measurements - Gross Components of Purchases, Sales, Issuances and Settlements (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Assets:
Purchases	$ 386,000,000	$ 99,000,000	$ 483,000,000	$ 426,000,000
Sales	(624,000,000)	(187,000,000)	(631,000,000)	(366,000,000)
Issuances and Settlements	(104,000,000)	(79,000,000)	(634,000,000)	(667,000,000)
Purchases, Sales, Issuances and Settlements, Net	(342,000,000)	(167,000,000)	(782,000,000)	(607,000,000)
Liabilities:
Purchases	0	(91,000,000)	0	(407,000,000)
Sales	0	3,000,000	0	7,000,000
Issuances and Settlements	(1,000,000)	131,000,000	345,000,000	(36,000,000)
Purchases, Sales, Issuances and Settlements, Net	(1,000,000)	43,000,000	345,000,000	(436,000,000)
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	0	(5,000,000)	(27,000,000)	(4,000,000)
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	0	0	1,000,000	(9,000,000)
Derivative liabilities, net
Liabilities:
Purchases	0	(91,000,000)	0	(407,000,000)
Sales	0	3,000,000	0	7,000,000
Issuances and Settlements	2,000,000	160,000,000	376,000,000	303,000,000
Purchases, Sales, Issuances and Settlements, Net	2,000,000	72,000,000	376,000,000	(97,000,000)
Fortitude Re funds withheld payable
Liabilities:
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	(3,000,000)	(29,000,000)	(10,000,000)	(339,000,000)
Purchases, Sales, Issuances and Settlements, Net	(3,000,000)	(29,000,000)	(10,000,000)	(339,000,000)
Other Liabilities
Liabilities:
Purchases			0
Sales			0
Issuances and Settlements			(21,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	0	(21,000,000)	0
Bonds available for sale
Assets:
Purchases	367,000,000	99,000,000	449,000,000	386,000,000
Sales	(624,000,000)	(175,000,000)	(630,000,000)	(354,000,000)
Issuances and Settlements	(88,000,000)	(85,000,000)	(457,000,000)	(647,000,000)
Purchases, Sales, Issuances and Settlements, Net	(345,000,000)	(161,000,000)	(638,000,000)	(615,000,000)
Bonds available for sale | Obligations of states, municipalities and political subdivisions
Assets:
Purchases		0	1,000,000	1,000,000
Sales		(11,000,000)	0	(15,000,000)
Issuances and Settlements		0	0	(2,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	(11,000,000)	1,000,000	(16,000,000)
Bonds available for sale | Non-U.S. governments
Assets:
Purchases		0	4,000,000	0
Sales		0	0	0
Issuances and Settlements		(3,000,000)	(4,000,000)	(5,000,000)
Purchases, Sales, Issuances and Settlements, Net		(3,000,000)	0	(5,000,000)
Bonds available for sale | Corporate debt
Assets:
Purchases	10,000,000	5,000,000	21,000,000	13,000,000
Sales	(4,000,000)	0	(7,000,000)	0
Issuances and Settlements	(2,000,000)	(68,000,000)	(70,000,000)	(423,000,000)
Purchases, Sales, Issuances and Settlements, Net	4,000,000	(63,000,000)	(56,000,000)	(410,000,000)
Bonds available for sale | RMBS
Assets:
Purchases	87,000,000	18,000,000	87,000,000	188,000,000
Sales	(45,000,000)	(6,000,000)	(46,000,000)	(25,000,000)
Issuances and Settlements	(65,000,000)	(75,000,000)	(214,000,000)	(244,000,000)
Purchases, Sales, Issuances and Settlements, Net	(23,000,000)	(63,000,000)	(173,000,000)	(81,000,000)
Bonds available for sale | CMBS
Assets:
Purchases	0	0	0	1,000,000
Sales	(12,000,000)	(7,000,000)	(12,000,000)	(12,000,000)
Issuances and Settlements	0	13,000,000	(18,000,000)	18,000,000
Purchases, Sales, Issuances and Settlements, Net	(12,000,000)	6,000,000	(30,000,000)	7,000,000
Bonds available for sale | CLO/ABS
Assets:
Purchases	270,000,000	76,000,000	336,000,000	183,000,000
Sales	(563,000,000)	(151,000,000)	(565,000,000)	(302,000,000)
Issuances and Settlements	(21,000,000)	48,000,000	(151,000,000)	9,000,000
Purchases, Sales, Issuances and Settlements, Net	(314,000,000)	(27,000,000)	(380,000,000)	(110,000,000)
Other bond securities
Assets:
Purchases	0	0	14,000,000	34,000,000
Sales	0	(10,000,000)	(1,000,000)	(10,000,000)
Issuances and Settlements	(3,000,000)	7,000,000	(11,000,000)	(11,000,000)
Purchases, Sales, Issuances and Settlements, Net	(3,000,000)	(3,000,000)	2,000,000	13,000,000
Other bond securities | Corporate debt
Assets:
Purchases				20,000,000
Sales				0
Issuances and Settlements				24,000,000
Purchases, Sales, Issuances and Settlements, Net	0	0	0	44,000,000
Other bond securities | RMBS
Assets:
Purchases	0	0	3,000,000	0
Sales	0	0	(1,000,000)	0
Issuances and Settlements	(2,000,000)	(2,000,000)	(4,000,000)	(17,000,000)
Purchases, Sales, Issuances and Settlements, Net	(2,000,000)	(2,000,000)	(2,000,000)	(17,000,000)
Other bond securities | CLO/ABS
Assets:
Purchases	0	0	11,000,000	14,000,000
Sales	0	(10,000,000)	0	(10,000,000)
Issuances and Settlements	(1,000,000)	9,000,000	(7,000,000)	(18,000,000)
Purchases, Sales, Issuances and Settlements, Net	(1,000,000)	(1,000,000)	4,000,000	(14,000,000)
Equity securities
Assets:
Purchases	2,000,000	0	2,000,000	5,000,000
Sales	0	(2,000,000)	0	(2,000,000)
Issuances and Settlements	0	0	0	(1,000,000)
Purchases, Sales, Issuances and Settlements, Net	2,000,000	(2,000,000)	2,000,000	2,000,000
Other invested assets
Assets:
Purchases	17,000,000	0	18,000,000	1,000,000
Sales	0	0	0	0
Issuances and Settlements	(12,000,000)	(2,000,000)	(52,000,000)	(13,000,000)
Purchases, Sales, Issuances and Settlements, Net	5,000,000	(2,000,000)	(34,000,000)	(12,000,000)
Other assets
Assets:
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	(1,000,000)	1,000,000	(114,000,000)	5,000,000
Purchases, Sales, Issuances and Settlements, Net	$ (1,000,000)	$ 1,000,000	$ (114,000,000)	$ 5,000,000