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Investments - Amortized Cost or Cost and Fair Value of Available for Sale Securities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024	Jun. 30, 2025	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 72,008	$ 66,195
Allowance for Credit Losses	(43)	(38)	$ (45)	$ (40)	$ (33)	$ (34)
Gross Unrealized Gains	1,203	962
Gross Unrealized Losses	(1,984)	(3,113)
Bonds available for sale	71,184	64,006
Non-Investment Grade
Debt Securities, Available-for-sale [Line Items]
Bonds available for sale	$ 5,800	$ 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	$ 3,837	$ 3,346
Allowance for Credit Losses	0	0
Gross Unrealized Gains	34	20
Gross Unrealized Losses	(84)	(99)
Bonds available for sale	3,787	3,267
Obligations of states, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,878	3,223
Allowance for Credit Losses	0	0
Gross Unrealized Gains	69	32
Gross Unrealized Losses	(58)	(112)
Bonds available for sale	2,889	3,143
Non-U.S. governments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,906	8,644
Allowance for Credit Losses	(1)	(1)
Gross Unrealized Gains	82	54
Gross Unrealized Losses	(374)	(590)
Bonds available for sale	6,613	8,107
Corporate debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,496	33,031
Allowance for Credit Losses	(38)	(28)
Gross Unrealized Gains	613	581
Gross Unrealized Losses	(1,120)	(1,758)
Bonds available for sale	36,951	31,826
Mortgage-backed, asset-backed and collateralized
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,891	17,951
Allowance for Credit Losses	(4)	(9)
Gross Unrealized Gains	405	275
Gross Unrealized Losses	(348)	(554)
Bonds available for sale	20,944	17,663
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,302	8,820
Allowance for Credit Losses	(4)	(6)
Gross Unrealized Gains	293	209
Gross Unrealized Losses	(288)	(419)
Bonds available for sale	10,303	8,604
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,165	3,988
Allowance for Credit Losses	0	(3)
Gross Unrealized Gains	63	32
Gross Unrealized Losses	(38)	(91)
Bonds available for sale	4,190	3,926
CLO/ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,424	5,143
Allowance for Credit Losses	0	0
Gross Unrealized Gains	49	34
Gross Unrealized Losses	(22)	(44)
Bonds available for sale	$ 6,451	$ 5,133