Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Accumulated Other Comprehensive Loss [Roll Forward]
Beginning Balance	$ 19,285	$ 13,384	$ 10,741
Other comprehensive income (loss)	501	(262)	(212)
Ending Balance	27,709	19,285	13,384
Foreign currency translation adjustments
Increase (Decrease) in Accumulated Other Comprehensive Loss [Roll Forward]
Beginning Balance	(1,001)	(722)	(512)
Other comprehensive income (loss)	533	(279)	(210)
Ending Balance	(468)	(1,001)	(722)
Unrealized gains on available-for-sale securities
Increase (Decrease) in Accumulated Other Comprehensive Loss [Roll Forward]
Beginning Balance	16	(1)	1
Other comprehensive income (loss)	(32)	17	(2)
Ending Balance	(16)	16	(1)
Total
Increase (Decrease) in Accumulated Other Comprehensive Loss [Roll Forward]
Beginning Balance	(985)	(723)	(511)
Other comprehensive income (loss)	501	(262)	(212)
Ending Balance	$ (484)	$ (985)	$ (723)