Fair Value Measurements - Gross Components of Purchases, Sales, Issuances and Settlements (Details) - USD ($)	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Assets:
Purchases	$ 160,000,000	$ 163,000,000
Sales	(51,000,000)	(52,000,000)
Issuances and Settlements	(213,000,000)	(237,000,000)
Purchases, Sales, Issuances and Settlements, Net	(104,000,000)	(126,000,000)
Liabilities:
Purchases	0	0
Sales	0	0
Issuances and Settlements	17,000,000	8,000,000
Purchases, Sales, Issuances and Settlements, Net	17,000,000	8,000,000
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	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	1,000,000	5,000,000
Fortitude Re funds withheld payable
Liabilities:
Purchases	0	0
Sales	0	0
Issuances and Settlements	17,000,000	8,000,000
Purchases, Sales, Issuances and Settlements, Net	17,000,000	8,000,000
Other Liabilities
Liabilities:
Purchases, Sales, Issuances and Settlements, Net	0	0
Bonds available for sale
Assets:
Purchases	143,000,000	149,000,000
Sales	(37,000,000)	(48,000,000)
Issuances and Settlements	(209,000,000)	(208,000,000)
Purchases, Sales, Issuances and Settlements, Net	(103,000,000)	(107,000,000)
Bonds available for sale | Obligations of states, municipalities and political subdivisions
Assets:
Purchases, Sales, Issuances and Settlements, Net	0	0
Bonds available for sale | Non-U.S. governments
Assets:
Purchases	3,000,000
Sales	(3,000,000)
Issuances and Settlements	(3,000,000)
Purchases, Sales, Issuances and Settlements, Net	(3,000,000)
Bonds available for sale | Corporate debt
Assets:
Purchases	4,000,000	3,000,000
Sales	(5,000,000)	(4,000,000)
Issuances and Settlements	0	(124,000,000)
Purchases, Sales, Issuances and Settlements, Net	(1,000,000)	(125,000,000)
Bonds available for sale | RMBS
Assets:
Purchases	9,000,000	0
Sales	0	(3,000,000)
Issuances and Settlements	(46,000,000)	(54,000,000)
Purchases, Sales, Issuances and Settlements, Net	(37,000,000)	(57,000,000)
Bonds available for sale | CMBS
Assets:
Purchases	0	0
Sales	(2,000,000)	(4,000,000)
Issuances and Settlements	(2,000,000)	0
Purchases, Sales, Issuances and Settlements, Net	(4,000,000)	(4,000,000)
Bonds available for sale | CLO/ABS
Assets:
Purchases	127,000,000	146,000,000
Sales	(27,000,000)	(37,000,000)
Issuances and Settlements	(158,000,000)	(30,000,000)
Purchases, Sales, Issuances and Settlements, Net	(58,000,000)	79,000,000
Other bond securities
Assets:
Purchases	0	0
Sales	0	0
Issuances and Settlements	(3,000,000)	(5,000,000)
Purchases, Sales, Issuances and Settlements, Net	(3,000,000)	(5,000,000)
Other bond securities | Corporate debt
Assets:
Purchases, Sales, Issuances and Settlements, Net		0
Other bond securities | RMBS
Assets:
Purchases	0	0
Sales	0	0
Issuances and Settlements	(1,000,000)	(1,000,000)
Purchases, Sales, Issuances and Settlements, Net	(1,000,000)	(1,000,000)
Other bond securities | CLO/ABS
Assets:
Purchases	0	0
Sales	0	0
Issuances and Settlements	(2,000,000)	(4,000,000)
Purchases, Sales, Issuances and Settlements, Net	(2,000,000)	(4,000,000)
Equity securities
Assets:
Purchases	16,000,000	14,000,000
Sales	(14,000,000)	(4,000,000)
Issuances and Settlements	0	0
Purchases, Sales, Issuances and Settlements, Net	2,000,000	10,000,000
Other invested assets
Assets:
Purchases	1,000,000	0
Sales	0	0
Issuances and Settlements	(1,000,000)	(24,000,000)
Purchases, Sales, Issuances and Settlements, Net	0	(24,000,000)
Other assets
Assets:
Purchases, Sales, Issuances and Settlements, Net	$ 0	$ 0