Income Taxes - Reconciliation of Tax Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross tax contingencies – beginning of period	$ 2,309	$ 1,710	$ 1,181
Gross increases to tax positions in prior periods	164	223	355
Gross decreases to tax positions in prior periods	(90)	(139)	(133)
Gross increases to current period tax positions	1,088	518	308
Settlements with tax authorities	(36)	0	0
Lapse of statute of limitations	(21)	(3)	(1)
Gross tax contingencies - end of period	3,414	$ 2,309	$ 1,710
Tax contingencies, that if fully recognized, would decrease our effective tax rate	$ 1,700