Stockholders' Equity Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 21,534	$ 20,501
Net current period other comprehensive income (loss)	50	(42)
Ending Balance	22,091	20,651
Foreign Currency Translation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	47	85
Reclassifications to retained earnings as a result of the adoption of new accounting standards			$ 0
Balance at January 1, 2018			85
Net unrealized gain (loss)	21	7
Reclassifications to net income	0	0
Net current period other comprehensive income (loss)	21	7
Ending Balance	68	92
Unrealized Gains and Losses on Available-for-Sale Securities
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(52)	194
Balance at January 1, 2018			(99)
Net unrealized gain (loss)	30	(36)
Reclassifications to net income	0	0
Net current period other comprehensive income (loss)	30	(36)
Ending Balance	(22)	(135)
Unrealized Gains and Losses on Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	85	(114)
Reclassifications to retained earnings as a result of the adoption of new accounting standards			0
Balance at January 1, 2018			(114)
Net unrealized gain (loss)	28	(61)
Reclassifications to net income	(29)	48
Net current period other comprehensive income (loss)	(1)	(13)
Ending Balance	84	(127)
AOCI Attributable to Parent
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	80	165
Reclassifications to retained earnings as a result of the adoption of new accounting standards			(293)
Balance at January 1, 2018			$ (128)
Net unrealized gain (loss)	79	(90)
Reclassifications to net income	(29)	48
Net current period other comprehensive income (loss)	50	(42)
Ending Balance	$ 130	$ (170)