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Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Values of Investments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt and Equity Securities
Amortized Cost	$ 177
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	177
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	38
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Debt Securities, Available-for-sale	38
Corporate bonds
Debt Securities
Amortized Cost	87
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Debt Securities, Available-for-sale	87
U.S. government agencies and government- sponsored agencies
Debt Securities
Amortized Cost	6
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Debt Securities, Available-for-sale	6
U.S. treasuries
Debt Securities
Amortized Cost	34
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Debt Securities, Available-for-sale	34
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	11
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Debt Securities, Available-for-sale	$ 11

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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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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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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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