Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Values of Investments (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt and Equity Securities
Amortized Cost	$ 197	$ 195
Gross Unrealized Gains	2	0
Gross Unrealized Losses	0	0
Fair Value	199	195
Asset-backed securities
Debt Securities
Amortized Cost	35	33
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	35	33
Corporate bonds
Debt Securities
Amortized Cost	94	99
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	95	99
U.S. government agencies and government- sponsored agencies
Debt Securities
Amortized Cost	5	7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	5	7
U.S. treasuries
Debt Securities
Amortized Cost	58	46
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	59	46
Exchange traded fund
Equity Securities
Amortized Cost	1	1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1	1
Other debt securities
Debt Securities
Amortized Cost	4	9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 4	$ 9

Debt Securities, Available-For-Sale, And Equity Securities, FV-NI

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Debt Securities, Available-For-Sale, And Equity Securities, FV-NI [Abstract]

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Debt Securities, Available-For-Sale, And Equity Securities, FV-NI, Cost

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Debt Securities, Available-For-Sale, And Equity Securities, FV-NI, Gross Unrealized Gains

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Debt Securities, Available-For-Sale, And Equity Securities, FV-NI, Gross Unrealized Loss

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Equity Securities, FV-NI [Abstract]

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Equity Securities, FV-NI, Accumulated Gross Unrealized Gain, Before Tax

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Equity Securities, FV-NI, Accumulated Gross Unrealized Loss, Before Tax

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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, 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).

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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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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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