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Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Values of Investments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities
Amortized Cost	$ 195
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	195
Available-for-Sale Securities
Amortized Cost		$ 38
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		38
Asset-backed securities
Debt Securities
Amortized Cost	33
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	33
Corporate bonds
Debt Securities
Amortized Cost	99
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	99
U.S. government agencies and government- sponsored agencies
Debt Securities
Amortized Cost	7
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	7
U.S. treasuries
Debt Securities
Amortized Cost	46
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	46
Available-for-Sale Securities
Amortized Cost		37
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		37
Exchange traded fund
Equity Securities
Amortized Cost	1
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	1
Available-for-Sale Securities
Amortized Cost		1
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		$ 1
Other debt securities
Debt Securities
Amortized Cost	9
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	$ 9

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

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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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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as 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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