Investments - Debt Securities (Details) $ in Millions	Dec. 31, 2018 USD ($)
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] (Deprecated 2018-01-31)
Total	$ 15,119
Gross Unrealized Gains	172
Gross Unrealized Losses	(36)
Fair Value	15,255
Supporting experience-rated products
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] (Deprecated 2018-01-31)
Gross Unrealized Gains	12
Gross Unrealized Losses	(2)
Fair Value	916
U.S. government securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] (Deprecated 2018-01-31)
Total	1,662
Gross Unrealized Gains	26
Gross Unrealized Losses	0
Fair Value	1,688
States, municipalities and political subdivisions
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] (Deprecated 2018-01-31)
Total	2,370
Gross Unrealized Gains	30
Gross Unrealized Losses	(1)
Fair Value	2,399
U.S. corporate securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] (Deprecated 2018-01-31)
Total	6,444
Gross Unrealized Gains	61
Gross Unrealized Losses	(16)
Fair Value	6,489
Foreign securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] (Deprecated 2018-01-31)
Total	2,355
Gross Unrealized Gains	31
Gross Unrealized Losses	(3)
Fair Value	2,383
Residential mortgage-backed securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] (Deprecated 2018-01-31)
Total	567
Gross Unrealized Gains	10
Gross Unrealized Losses	0
Fair Value	577
Commercial mortgage-backed securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] (Deprecated 2018-01-31)
Total	594
Gross Unrealized Gains	11
Gross Unrealized Losses	0
Fair Value	605
Other asset-backed securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] (Deprecated 2018-01-31)
Total	1,097
Gross Unrealized Gains	3
Gross Unrealized Losses	(15)
Fair Value	1,085
Redeemable preferred securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] (Deprecated 2018-01-31)
Total	30
Gross Unrealized Gains	0
Gross Unrealized Losses	(1)
Fair Value	$ 29

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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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Amount, before tax, of unrealized loss 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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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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