Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Values of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt and Equity Securities
Amortized Cost	$ 174
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	174
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	35
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	35
Corporate bonds
Debt Securities
Amortized Cost	86
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	86
U.S. government agencies and government- sponsored agencies
Debt Securities
Amortized Cost	8
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	8
U.S. treasuries
Debt Securities
Amortized Cost	34
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	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	10
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	$ 10

Available-For-Sale Debt And Equity Securities [Abstract]

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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 2
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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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 before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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No definition available.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
-Paragraph 1
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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
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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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