Stockholders' Equity - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 21,534	$ 20,501	$ 19,363
Other comprehensive income (loss)	5	208	(113)
Ending Balance	22,650	21,534	20,501
Foreign Currency Translation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	47	85
Cumulative effect from the adoption of new accounting standards				$ 0
Balance at January 1, 2018				85
Net unrealized (loss) gain	6	(38)
Reclassifications to net income	0	0
Other comprehensive income (loss)	6	(38)
Ending Balance	53	47	85
Unrealized Gains and Losses on Available-for-Sale Securities
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(52)	194
Cumulative effect from the adoption of new accounting standards				(293)
Balance at January 1, 2018				(99)
Net unrealized (loss) gain	54	43
Reclassifications to net income	(1)	4
Other comprehensive income (loss)	53	47
Ending Balance	1	(52)	194
Unrealized Gains and Losses on Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	85	(114)
Cumulative effect from the adoption of new accounting standards				0
Balance at January 1, 2018				(114)
Net unrealized (loss) gain	72	112
Reclassifications to net income	(126)	87
Other comprehensive income (loss)	(54)	199
Ending Balance	31	85	(114)
AOCI Attributable to Parent
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	80	165	278
Cumulative effect from the adoption of new accounting standards				(293)
Balance at January 1, 2018				$ (128)
Net unrealized (loss) gain	132	117
Reclassifications to net income	(127)	91
Other comprehensive income (loss)	5	208	(113)
Ending Balance	$ 85	$ 80	$ 165