Investments - Amortized Cost or Cost and Fair Value of Available for Sale Securities (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2025	Dec. 31, 2024	Mar. 31, 2025	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 70,229	$ 66,195
Allowance for Credit Losses	(45)	(38)	$ (30)	$ (33)	$ (28)	$ (34)
Gross Unrealized Gains	1,016	962
Gross Unrealized Losses	(2,340)	(3,113)
Bonds available for sale	68,860	64,006
Non-Investment Grade
Debt Securities, Available-for-sale [Line Items]
Bonds available for sale	$ 5,700	$ 3,600
Non-Investment Grade | Credit Concentration Risk | Bonds available for sale
Debt Securities, Available-for-sale [Line Items]
Concentration risk (as a percent)	8.00%	6.00%
U.S. government and government sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,964	$ 3,346
Allowance for Credit Losses	0	0
Gross Unrealized Gains	33	20
Gross Unrealized Losses	(88)	(99)
Bonds available for sale	2,909	3,267
Obligations of states, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,047	3,223
Allowance for Credit Losses	0	0
Gross Unrealized Gains	52	32
Gross Unrealized Losses	(76)	(112)
Bonds available for sale	3,023	3,143
Non-U.S. governments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,250	8,644
Allowance for Credit Losses	(1)	(1)
Gross Unrealized Gains	88	54
Gross Unrealized Losses	(424)	(590)
Bonds available for sale	6,913	8,107
Corporate debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,369	33,031
Allowance for Credit Losses	(34)	(28)
Gross Unrealized Gains	499	581
Gross Unrealized Losses	(1,349)	(1,758)
Bonds available for sale	36,485	31,826
Mortgage-backed, asset-backed and collateralized
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,599	17,951
Allowance for Credit Losses	(10)	(9)
Gross Unrealized Gains	344	275
Gross Unrealized Losses	(403)	(554)
Bonds available for sale	19,530	17,663
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,806	8,820
Allowance for Credit Losses	(5)	(6)
Gross Unrealized Gains	245	209
Gross Unrealized Losses	(329)	(419)
Bonds available for sale	9,717	8,604
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,741	3,988
Allowance for Credit Losses	(5)	(3)
Gross Unrealized Gains	52	32
Gross Unrealized Losses	(45)	(91)
Bonds available for sale	3,743	3,926
CLO/ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,052	5,143
Allowance for Credit Losses	0	0
Gross Unrealized Gains	47	34
Gross Unrealized Losses	(29)	(44)
Bonds available for sale	$ 6,070	$ 5,133