Income Taxes - Reconciliation of Tax Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross tax contingencies – beginning of period	$ 1,710	$ 1,181	$ 710
Gross increases to tax positions in prior periods	223	355	254
Gross decreases to tax positions in prior periods	(139)	(133)	(22)
Gross increases to current period tax positions	518	308	242
Lapse of statute of limitations	(3)	(1)	(3)
Gross tax contingencies - end of period	2,309	$ 1,710	$ 1,181
Tax contingencies, that if fully recognized, would decrease our effective tax rate	$ 1,200