Income Taxes (Reconciliation of Provision for Income Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Expected provision (benefit) at federal statutory rate	$ (568)	$ 101	$ 113
Increase (decrease) resulting from:
State income taxes, net of federal income tax effect	(55)	11	13
Change in valuation allowance	1,034	(522)	(121)
Removal of sequestration fee on AMT receivables	0	0	(5)
Other	(4)	(1)	1
Provision (Benefit) for Income Taxes	$ 407	$ (411)	$ 1