Income Taxes - Reconciliation of Total Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of unrecognized tax benefits
Unrecognized tax benefits balance at January 1	$ 1,037	$ 926	$ 1,138
Gross increase for tax positions taken during a prior year	97	151	103
Gross decrease for tax positions taken during a prior year	(1)	(168)	(123)
Gross increase for tax positions taken during the current year	38	291	132
Settlement with taxing authorities	(3)	(163)	(312)
Lapse of statute of limitations	(2)	0	(12)
Unrecognized tax benefits balance at December 31	$ 1,166	$ 1,037	$ 926