Stockholders' Equity - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 19,363	$ 19,113	$ 15,819
Other comprehensive income (loss)	(113)	190	(213)
Ending Balance	20,501	19,363	19,113
Foreign Currency Translation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	132	(45)
Other comprehensive income (loss) before reclassifications	(47)	177
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income (loss)	(47)	177
Ending Balance	85	132	(45)
Unrealized Gains and Losses on Available-for-Sale Securities
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(16)	(16)
Other comprehensive income (loss) before reclassifications	218	7
Amounts reclassified from accumulated other comprehensive income	(8)	(7)
Other comprehensive income (loss)	210	0
Ending Balance	194	(16)	(16)
Unrealized Gains and Losses on Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	162	149
Other comprehensive income (loss) before reclassifications	(304)	5
Amounts reclassified from accumulated other comprehensive income	28	8
Other comprehensive income (loss)	(276)	13
Ending Balance	(114)	162	149
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	278	88	301
Other comprehensive income (loss) before reclassifications	(133)	189
Amounts reclassified from accumulated other comprehensive income	20	1
Other comprehensive income (loss)	(113)	190	(213)
Ending Balance	$ 165	$ 278	$ 88