Investments - Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 26,433	$ 22,282
Gross Unrealized Gains	510	267
Gross Unrealized Losses	(453)	(836)
Fair Value	26,490	21,713
Allowance for expected credit losses	0	0
Supporting experience-rated products
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	14	10
Gross Unrealized Losses	(16)	(28)
Fair Value	548	592
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,755	2,071
Gross Unrealized Gains	52	19
Gross Unrealized Losses	(23)	(54)
Fair Value	2,784	2,036
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	738	2,219
Gross Unrealized Gains	11	31
Gross Unrealized Losses	(11)	(35)
Fair Value	738	2,215
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,086	10,156
Gross Unrealized Gains	283	133
Gross Unrealized Losses	(254)	(446)
Fair Value	13,115	9,843
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,645	2,593
Gross Unrealized Gains	68	41
Gross Unrealized Losses	(69)	(122)
Fair Value	2,644	2,512
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	815	862
Gross Unrealized Gains	13	8
Gross Unrealized Losses	(33)	(60)
Fair Value	795	810
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,609	1,066
Gross Unrealized Gains	32	9
Gross Unrealized Losses	(58)	(100)
Fair Value	1,583	975
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,767	3,294
Gross Unrealized Gains	51	26
Gross Unrealized Losses	(5)	(18)
Fair Value	4,813	3,302
Redeemable preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18	21
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Fair Value	$ 18	$ 20