Investments - Unrealized Loss Position (Details) $ in Millions	Sep. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| security	648	2,279
Fair Value	$ 790	$ 2,409
Unrealized Losses	$ 13	$ 36
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| security	8	8
Fair Value	$ 41	$ 26
Unrealized Losses	$ 0	$ 0
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| security	38	54
Fair Value	$ 53	$ 86
Unrealized Losses	$ 1	$ 1
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| security	217	1,399
Fair Value	$ 254	$ 1,431
Unrealized Losses	$ 7	$ 16
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| security	27	243
Fair Value	$ 39	$ 314
Unrealized Losses	$ 0	$ 3
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| security	31	45
Fair Value	$ 17	$ 1
Unrealized Losses	$ 0	$ 0
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| security	327	516
Fair Value	$ 386	$ 528
Unrealized Losses	$ 5	$ 15
Redeemable preferred securities
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| security		14
Fair Value		$ 23
Unrealized Losses		$ 1

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922888-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118942371&loc=SL82922954-210456

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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