Investments - Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 26,345	$ 26,562
Gross Unrealized Gains	246	178
Gross Unrealized Losses	(547)	(707)
Fair Value	26,044	26,033
Allowance for expected credit losses	0	0
Supporting experience-rated products
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	6	5
Gross Unrealized Losses	(22)	(30)
Fair Value	517	543
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,869	2,826
Gross Unrealized Gains	30	7
Gross Unrealized Losses	(19)	(38)
Fair Value	2,880	2,795
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	387	712
Gross Unrealized Gains	2	4
Gross Unrealized Losses	(13)	(18)
Fair Value	376	698
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,326	13,043
Gross Unrealized Gains	135	94
Gross Unrealized Losses	(334)	(412)
Fair Value	13,127	12,725
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,603	2,608
Gross Unrealized Gains	34	27
Gross Unrealized Losses	(81)	(111)
Fair Value	2,556	2,524
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	828	792
Gross Unrealized Gains	7	2
Gross Unrealized Losses	(41)	(54)
Fair Value	794	740
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,775	1,731
Gross Unrealized Gains	16	9
Gross Unrealized Losses	(50)	(67)
Fair Value	1,741	1,673
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,542	4,834
Gross Unrealized Gains	22	35
Gross Unrealized Losses	(9)	(7)
Fair Value	4,555	4,862
Redeemable preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15	16
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 15	$ 16