Income Taxes (Reconciliation of Changes to the Valuation Allowance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Asset, Valuation Allowance [Roll Forward]
Valuation allowance, beginning balance	$ 87	$ 609
Release of valuation allowance	0	(522)
Establishment of valuation allowance on opening deferred balance	408	0
Opening balance adjustments	6	0
Changes based on 2020 activity	626	0
Purchase accounting	412	0
Valuation allowance, ending balance	$ 1,539	$ 87