Fair Value Measurements - Gross Components of Purchases, Sales, Issuances and Settlements (Details) - USD ($)	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Assets:
Purchases	$ 163,000,000	$ 82,000,000
Sales	(52,000,000)	(6,000,000)
Issuances and Settlements	(237,000,000)	(314,000,000)
Purchases, Sales, Issuances and Settlements, Net	(126,000,000)	(238,000,000)
Liabilities:
Purchases	0	0
Sales	0	0
Issuances and Settlements	8,000,000	238,000,000
Purchases, Sales, Issuances and Settlements, Net	8,000,000	238,000,000
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	2,000,000	(3,000,000)
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	5,000,000	0
Derivative liabilities, net
Liabilities:
Purchases		0
Sales		0
Issuances and Settlements		218,000,000
Purchases, Sales, Issuances and Settlements, Net		218,000,000
Fortitude Re funds withheld payable
Liabilities:
Purchases	0	0
Sales	0	0
Issuances and Settlements	8,000,000	20,000,000
Purchases, Sales, Issuances and Settlements, Net	8,000,000	20,000,000
Bonds available for sale
Assets:
Purchases	149,000,000	67,000,000
Sales	(48,000,000)	(6,000,000)
Issuances and Settlements	(208,000,000)	(192,000,000)
Purchases, Sales, Issuances and Settlements, Net	(107,000,000)	(131,000,000)
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	1,000,000
Bonds available for sale | Corporate debt
Assets:
Purchases	3,000,000	6,000,000
Sales	(4,000,000)	(3,000,000)
Issuances and Settlements	(124,000,000)	(41,000,000)
Purchases, Sales, Issuances and Settlements, Net	(125,000,000)	(38,000,000)
Bonds available for sale | RMBS
Assets:
Purchases	0	0
Sales	(3,000,000)	(1,000,000)
Issuances and Settlements	(54,000,000)	(74,000,000)
Purchases, Sales, Issuances and Settlements, Net	(57,000,000)	(75,000,000)
Bonds available for sale | CMBS
Assets:
Purchases	0
Sales	(4,000,000)
Issuances and Settlements	0
Purchases, Sales, Issuances and Settlements, Net	(4,000,000)	0
Bonds available for sale | CLO/ABS
Assets:
Purchases	146,000,000	60,000,000
Sales	(37,000,000)	(2,000,000)
Issuances and Settlements	(30,000,000)	(77,000,000)
Purchases, Sales, Issuances and Settlements, Net	79,000,000	(19,000,000)
Other bond securities
Assets:
Purchases	0	14,000,000
Sales	0	0
Issuances and Settlements	(5,000,000)	(4,000,000)
Purchases, Sales, Issuances and Settlements, Net	(5,000,000)	10,000,000
Other bond securities | Corporate debt
Assets:
Purchases, Sales, Issuances and Settlements, Net	0	0
Other bond securities | RMBS
Assets:
Purchases		3,000,000
Sales		0
Issuances and Settlements	(1,000,000)	(1,000,000)
Purchases, Sales, Issuances and Settlements, Net	(1,000,000)	2,000,000
Other bond securities | CLO/ABS
Assets:
Purchases	0	11,000,000
Sales	0	0
Issuances and Settlements	(4,000,000)	(3,000,000)
Purchases, Sales, Issuances and Settlements, Net	(4,000,000)	8,000,000
Equity securities
Assets:
Purchases	14,000,000
Sales	(4,000,000)
Issuances and Settlements	0
Purchases, Sales, Issuances and Settlements, Net	10,000,000	0
Other invested assets
Assets:
Purchases	0	1,000,000
Sales	0	0
Issuances and Settlements	(24,000,000)	(4,000,000)
Purchases, Sales, Issuances and Settlements, Net	(24,000,000)	(3,000,000)
Other assets
Assets:
Purchases		0
Sales		0
Issuances and Settlements		(114,000,000)
Purchases, Sales, Issuances and Settlements, Net	$ 0	$ (114,000,000)