Fair Value Measurements - Gross Components of Purchases, Sales, Issuances and Settlements (Details) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2024	Dec. 31, 2023
Assets:
Purchases			$ 609,000,000	$ 713,000,000
Sales			(781,000,000)	(516,000,000)
Issuances and Settlements			(754,000,000)	(987,000,000)
Purchases, Sales, Issuances and Settlements, Net			(926,000,000)	(790,000,000)
Liabilities:
Purchases			0	(481,000,000)
Sales			0	7,000,000
Issuances and Settlements			302,000,000	196,000,000
Purchases, Sales, Issuances and Settlements, Net			302,000,000	(278,000,000)
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			(35,000,000)	2,000,000
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			(13,000,000)	(11,000,000)
Derivative liabilities, net
Liabilities:
Purchases			0	(481,000,000)
Sales			0	7,000,000
Issuances and Settlements			377,000,000	576,000,000
Purchases, Sales, Issuances and Settlements, Net			377,000,000	102,000,000
Fortitude Re funds withheld payable
Liabilities:
Purchases			0	0
Sales			0	0
Issuances and Settlements			(55,000,000)	(380,000,000)
Purchases, Sales, Issuances and Settlements, Net			(55,000,000)	(380,000,000)
Other Liabilities
Liabilities:
Purchases			0
Sales			0
Issuances and Settlements			(20,000,000)
Purchases, Sales, Issuances and Settlements, Net			(20,000,000)	0
Bonds available for sale
Assets:
Purchases			584,000,000	542,000,000
Sales			(778,000,000)	(504,000,000)
Issuances and Settlements			(588,000,000)	(923,000,000)
Purchases, Sales, Issuances and Settlements, Net			(782,000,000)	(885,000,000)
Bonds available for sale | Obligations of states, municipalities and political subdivisions
Assets:
Purchases			1,000,000	1,000,000
Sales			0	(15,000,000)
Issuances and Settlements			(1,000,000)	(2,000,000)
Purchases, Sales, Issuances and Settlements, Net			0	(16,000,000)
Bonds available for sale | Non-U.S. governments
Assets:
Purchases			4,000,000	0
Sales			0	0
Issuances and Settlements			(4,000,000)	(5,000,000)
Purchases, Sales, Issuances and Settlements, Net			0	(5,000,000)
Bonds available for sale | Corporate debt
Assets:
Purchases			43,000,000	21,000,000
Sales			(29,000,000)	(4,000,000)
Issuances and Settlements			(85,000,000)	(554,000,000)
Purchases, Sales, Issuances and Settlements, Net			(71,000,000)	(537,000,000)
Bonds available for sale | RMBS
Assets:
Purchases			89,000,000	228,000,000
Sales			(53,000,000)	(25,000,000)
Issuances and Settlements			(274,000,000)	(345,000,000)
Purchases, Sales, Issuances and Settlements, Net			(238,000,000)	(142,000,000)
Bonds available for sale | CMBS
Assets:
Purchases			0	1,000,000
Sales			(15,000,000)	(23,000,000)
Issuances and Settlements			(17,000,000)	16,000,000
Purchases, Sales, Issuances and Settlements, Net			(32,000,000)	(6,000,000)
Bonds available for sale | CLO/ABS
Assets:
Purchases			447,000,000	291,000,000
Sales			(681,000,000)	(437,000,000)
Issuances and Settlements			(207,000,000)	(33,000,000)
Purchases, Sales, Issuances and Settlements, Net			(441,000,000)	(179,000,000)
Other bond securities
Assets:
Purchases			16,000,000	34,000,000
Sales			(1,000,000)	(10,000,000)
Issuances and Settlements			(14,000,000)	(32,000,000)
Purchases, Sales, Issuances and Settlements, Net			1,000,000	(8,000,000)
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	44,000,000
Other bond securities | RMBS
Assets:
Purchases			3,000,000	0
Sales			(1,000,000)	0
Issuances and Settlements			(5,000,000)	(18,000,000)
Purchases, Sales, Issuances and Settlements, Net			(3,000,000)	(18,000,000)
Other bond securities | CLO/ABS
Assets:
Purchases			13,000,000	14,000,000
Sales			0	(10,000,000)
Issuances and Settlements			(9,000,000)	(38,000,000)
Purchases, Sales, Issuances and Settlements, Net			4,000,000	(34,000,000)
Equity securities
Assets:
Purchases			6,000,000	6,000,000
Sales			(2,000,000)	(2,000,000)
Issuances and Settlements			0	(1,000,000)
Purchases, Sales, Issuances and Settlements, Net			4,000,000	3,000,000
Other invested assets
Assets:
Purchases			3,000,000	1,000,000
Sales			0	0
Issuances and Settlements			(38,000,000)	(37,000,000)
Purchases, Sales, Issuances and Settlements, Net			(35,000,000)	(36,000,000)
Other assets
Assets:
Purchases			0	130,000,000
Sales			0	0
Issuances and Settlements			(114,000,000)	6,000,000
Purchases, Sales, Issuances and Settlements, Net			$ (114,000,000)	$ 136,000,000