Fair Value Measurements - Gross Components of Purchases, Sales, Issuances and Settlements (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Assets:
Purchases	$ 524,000,000	$ 386,000,000	$ 1,106,000,000	$ 483,000,000
Sales	(32,000,000)	(624,000,000)	(132,000,000)	(631,000,000)
Issuances and Settlements	(205,000,000)	(104,000,000)	(549,000,000)	(634,000,000)
Purchases, Sales, Issuances and Settlements, Net	287,000,000	(342,000,000)	425,000,000	(782,000,000)
Liabilities:
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	28,000,000	(1,000,000)	(84,000,000)	345,000,000
Purchases, Sales, Issuances and Settlements, Net	28,000,000	(1,000,000)	(84,000,000)	345,000,000
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	0	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	0	0	6,000,000	1,000,000
Derivative liabilities, net
Liabilities:
Purchases		0		0
Sales		0		0
Issuances and Settlements		2,000,000		376,000,000
Purchases, Sales, Issuances and Settlements, Net		2,000,000		376,000,000
Fortitude Re funds withheld payable
Liabilities:
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	28,000,000	(3,000,000)	(84,000,000)	(10,000,000)
Purchases, Sales, Issuances and Settlements, Net	28,000,000	(3,000,000)	(84,000,000)	(10,000,000)
Other Liabilities
Liabilities:
Purchases				0
Sales				0
Issuances and Settlements				(21,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	0	0	(21,000,000)
Bonds available for sale
Assets:
Purchases	483,000,000	367,000,000	1,028,000,000	449,000,000
Sales	(16,000,000)	(624,000,000)	(99,000,000)	(630,000,000)
Issuances and Settlements	(203,000,000)	(88,000,000)	(510,000,000)	(457,000,000)
Purchases, Sales, Issuances and Settlements, Net	264,000,000	(345,000,000)	419,000,000	(638,000,000)
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
Sales			0	0
Issuances and Settlements			(1,000,000)	(4,000,000)
Purchases, Sales, Issuances and Settlements, Net			(1,000,000)	0
Bonds available for sale | Corporate debt
Assets:
Purchases	0	10,000,000	5,000,000	21,000,000
Sales	(2,000,000)	(4,000,000)	(8,000,000)	(7,000,000)
Issuances and Settlements	(2,000,000)	(2,000,000)	(152,000,000)	(70,000,000)
Purchases, Sales, Issuances and Settlements, Net	(4,000,000)	4,000,000	(155,000,000)	(56,000,000)
Bonds available for sale | RMBS
Assets:
Purchases	46,000,000	87,000,000	54,000,000	87,000,000
Sales	0	(45,000,000)	(3,000,000)	(46,000,000)
Issuances and Settlements	(51,000,000)	(65,000,000)	(152,000,000)	(214,000,000)
Purchases, Sales, Issuances and Settlements, Net	(5,000,000)	(23,000,000)	(101,000,000)	(173,000,000)
Bonds available for sale | CMBS
Assets:
Purchases	0	0	0	0
Sales	0	(12,000,000)	(4,000,000)	(12,000,000)
Issuances and Settlements	(4,000,000)	0	(5,000,000)	(18,000,000)
Purchases, Sales, Issuances and Settlements, Net	(4,000,000)	(12,000,000)	(9,000,000)	(30,000,000)
Bonds available for sale | CLO/ABS
Assets:
Purchases	437,000,000	270,000,000	969,000,000	336,000,000
Sales	(14,000,000)	(563,000,000)	(84,000,000)	(565,000,000)
Issuances and Settlements	(146,000,000)	(21,000,000)	(200,000,000)	(151,000,000)
Purchases, Sales, Issuances and Settlements, Net	277,000,000	(314,000,000)	685,000,000	(380,000,000)
Other bond securities
Assets:
Purchases	14,000,000	0	15,000,000	14,000,000
Sales	0	0	0	(1,000,000)
Issuances and Settlements	(1,000,000)	(3,000,000)	(8,000,000)	(11,000,000)
Purchases, Sales, Issuances and Settlements, Net	13,000,000	(3,000,000)	7,000,000	2,000,000
Other bond securities | Corporate debt
Assets:
Purchases, Sales, Issuances and Settlements, Net	0	0	0	0
Other bond securities | RMBS
Assets:
Purchases	3,000,000	0	3,000,000	3,000,000
Sales	0	0	0	(1,000,000)
Issuances and Settlements	0	(2,000,000)	(2,000,000)	(4,000,000)
Purchases, Sales, Issuances and Settlements, Net	3,000,000	(2,000,000)	1,000,000	(2,000,000)
Other bond securities | CLO/ABS
Assets:
Purchases	11,000,000	0	12,000,000	11,000,000
Sales	0	0	0	0
Issuances and Settlements	(1,000,000)	(1,000,000)	(6,000,000)	(7,000,000)
Purchases, Sales, Issuances and Settlements, Net	10,000,000	(1,000,000)	6,000,000	4,000,000
Equity securities
Assets:
Purchases	21,000,000	2,000,000	56,000,000	2,000,000
Sales	(16,000,000)	0	(33,000,000)	0
Issuances and Settlements	0	0	0	0
Purchases, Sales, Issuances and Settlements, Net	5,000,000	2,000,000	23,000,000	2,000,000
Other invested assets
Assets:
Purchases	6,000,000	17,000,000	7,000,000	18,000,000
Sales	0	0	0	0
Issuances and Settlements	(1,000,000)	(12,000,000)	(31,000,000)	(52,000,000)
Purchases, Sales, Issuances and Settlements, Net	5,000,000	5,000,000	(24,000,000)	(34,000,000)
Other assets
Assets:
Purchases		0		0
Sales		0		0
Issuances and Settlements		(1,000,000)		(114,000,000)
Purchases, Sales, Issuances and Settlements, Net	$ 0	$ (1,000,000)	$ 0	$ (114,000,000)