Stockholders' Equity - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 20,501	$ 19,363	$ 19,113
Other comprehensive income (loss)	208	(113)	190
Ending Balance	21,534	20,501	19,363
Foreign Currency Translation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	85	132
Cumulative effect from the adoption of new accounting standards				$ 0
Balance at January 1, 2018				85
Other comprehensive income (loss) before reclassifications	(38)	(47)
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income (loss)	(38)	(47)
Ending Balance	47	85	132
Unrealized Gains and Losses on Available-for-Sale Securities
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	194	(16)
Cumulative effect from the adoption of new accounting standards				(293)
Balance at January 1, 2018				(99)
Other comprehensive income (loss) before reclassifications	43	218
Amounts reclassified from accumulated other comprehensive income	4	(8)
Other comprehensive income (loss)	47	210
Ending Balance	(52)	194	(16)
Unrealized Gains and Losses on Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(114)	162
Cumulative effect from the adoption of new accounting standards				0
Balance at January 1, 2018				(114)
Other comprehensive income (loss) before reclassifications	112	(304)
Amounts reclassified from accumulated other comprehensive income	87	28
Other comprehensive income (loss)	199	(276)
Ending Balance	85	(114)	162
AOCI Attributable to Parent
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	165	278	88
Cumulative effect from the adoption of new accounting standards				(293)
Balance at January 1, 2018				$ (128)
Other comprehensive income (loss) before reclassifications	117	(133)
Amounts reclassified from accumulated other comprehensive income	91	20
Other comprehensive income (loss)	208	(113)	190
Ending Balance	$ 80	$ 165	$ 278