Investments - Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 23,308	$ 22,282
Gross Unrealized Gains	191	267
Gross Unrealized Losses	(873)	(836)
Fair Value	22,626	21,713
Allowance for expected credit losses	0	0
Supporting experience-rated products
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	7	10
Gross Unrealized Losses	(32)	(28)
Fair Value	552	592
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,205	2,071
Gross Unrealized Gains	8	19
Gross Unrealized Losses	(66)	(54)
Fair Value	2,147	2,036
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,029	2,219
Gross Unrealized Gains	18	31
Gross Unrealized Losses	(39)	(35)
Fair Value	2,008	2,215
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,951	10,156
Gross Unrealized Gains	94	133
Gross Unrealized Losses	(474)	(446)
Fair Value	10,571	9,843
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,594	2,593
Gross Unrealized Gains	33	41
Gross Unrealized Losses	(124)	(122)
Fair Value	2,503	2,512
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	857	862
Gross Unrealized Gains	5	8
Gross Unrealized Losses	(62)	(60)
Fair Value	800	810
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,121	1,066
Gross Unrealized Gains	8	9
Gross Unrealized Losses	(93)	(100)
Fair Value	1,036	975
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,533	3,294
Gross Unrealized Gains	25	26
Gross Unrealized Losses	(15)	(18)
Fair Value	3,543	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