Investments - Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 20,349	$ 19,396
Gross Unrealized Gains	1,230	1,803
Gross Unrealized Losses	(75)	(11)
Fair Value	21,504	21,188
Allowance for Credit Losses	0	0
Supporting experience-rated products
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	90	135
Gross Unrealized Losses	(2)	0
Fair Value	870	919
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,236	2,341
Gross Unrealized Gains	72	128
Gross Unrealized Losses	(2)	0
Fair Value	2,306	2,469
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,778	2,556
Gross Unrealized Gains	142	172
Gross Unrealized Losses	(4)	0
Fair Value	2,916	2,728
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,671	7,879
Gross Unrealized Gains	689	1,023
Gross Unrealized Losses	(36)	(8)
Fair Value	9,324	8,894
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,655	2,595
Gross Unrealized Gains	215	324
Gross Unrealized Losses	(12)	(1)
Fair Value	2,858	2,918
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	744	673
Gross Unrealized Gains	24	32
Gross Unrealized Losses	(6)	0
Fair Value	762	705
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	943	962
Gross Unrealized Gains	55	84
Gross Unrealized Losses	(13)	0
Fair Value	985	1,046
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,295	2,369
Gross Unrealized Gains	30	36
Gross Unrealized Losses	(2)	(2)
Fair Value	2,323	2,403
Redeemable preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27	21
Gross Unrealized Gains	3	4
Gross Unrealized Losses	0	0
Fair Value	$ 30	$ 25

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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