Income Taxes - A Reconciliation of Beginning and Ending Amounts of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 506	$ 441	$ 461
Additions for tax positions related to the current year	9	9	1
Additions for tax positions of prior years	7	56	18
Reductions for tax positions of prior year	(25)	0	(18)
Reductions for settlement	0	0	(18)
Reductions due to statute expiration	0	0	(3)
Ending balance	$ 497	$ 506	$ 441