Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at beginning of year	$ 1,538	$ 1,587
Increase in tax positions of the current year netted against deferred tax assets	18	11
Increase in tax positions of prior periods netted against deferred tax assets	5	6
Decrease in tax positions of the current year netted against deferred tax assets	(86)	(49)
Decrease in tax positions of prior periods netted against deferred tax assets	(5)	(19)
Increase in tax positions taken in the current year	4	5
Increase in tax positions taken in the prior year	1	10
Decrease due to payments/settlements	(1)	(8)
Decrease due to the reversal of tax positions taken in a prior year	0	(5)
Unrecognized tax benefits at end of year	$ 1,474	$ 1,538