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Stockholders' Equity - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 20,501
Cumulative effect from the adoption of new accounting standards		$ 190
Ending Balance	20,651
Foreign Currency Translation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	85
Net unrealized gain (loss)	7
Reclassifications to retained earnings as a result of the adoption of new accounting standards	0
Cumulative effect from the adoption of new accounting standards			$ 0
Net current period other comprehensive income (loss)	7
Ending Balance	92
Unrealized Gains and Losses on Available-for-Sale Securities
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	194
Net unrealized gain (loss)	(36)
Reclassifications to retained earnings as a result of the adoption of new accounting standards	0
Cumulative effect from the adoption of new accounting standards			(293)
Net current period other comprehensive income (loss)	(329)
Ending Balance	(135)
Unrealized Gains and Losses on Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(114)
Net unrealized gain (loss)	(61)
Reclassifications to retained earnings as a result of the adoption of new accounting standards	48
Cumulative effect from the adoption of new accounting standards			0
Net current period other comprehensive income (loss)	(13)
Ending Balance	(127)
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	165
Net unrealized gain (loss)	(90)
Reclassifications to retained earnings as a result of the adoption of new accounting standards	48
Cumulative effect from the adoption of new accounting standards		$ (293)	$ (293)
Net current period other comprehensive income (loss)	(335)
Ending Balance	$ (170)

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- Definition

Other Comprehensive Income (Loss), Including Cumulative Effect Of New Accounting Principle In Period Of Adoption, Net Of Tax

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.

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