Valuation of Debt and Equity Investments and Certain Liabilities - Additional Information (Details) - USD ($)	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Credit losses related to available-for-sale investments	$ 0	$ 0
Proceeds from sales, redemptions and maturities of short-term available-for-sale securities	1,584,000,000	$ 1,455,000,000
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets fair value	4,051,000,000		$ 4,229,000,000
Total liabilities fair value	266,000,000		246,000,000
Fair Value, Measurements, Recurring \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets fair value	0		0
Total liabilities fair value	$ 0		$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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Fair value of financial and nonfinancial obligations.

+ References

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- Definition

Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=109979856&loc=SL6284422-111562

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- Definition

The cash inflow from sales, maturities, prepayments, calls and collections of all investments, including securities and other assets, having ready marketability and intended by management to be liquidated, if necessary, within the current operating cycle. Includes cash flows from securities classified as trading securities that were acquired for reasons other than sale in the short-term.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

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