Income Taxes - Reconciliation of Total Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of unrecognized tax benefits
Unrecognized tax benefits balance at January 1	$ 846	$ 552	$ 419
Gross increase for tax positions of prior-years	66	89	8
Gross decrease for tax positions of prior-years	0	(17)	(11)
Gross increase for tax positions of current year	229	240	136
Settlement with taxing authorities	(1)	(18)	0
Lapse of statute of limitations	(2)	0	0
Unrecognized tax benefits balance at December 31	$ 1,138	$ 846	$ 552