Income Taxes (Components of Deferred Tax Balances) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax liabilities:
Differences between book and tax basis of property	$ 0	$ 312
Derivative activity	0	34
Right of use lease asset	38	37
Other	2	2
Total deferred tax liabilities	40	385
Deferred tax assets:
Differences between book and tax basis of property	295	0
Accrued compensation	38	33
Accrued pension costs	11	9
Asset retirement obligations	20	13
Net operating loss carryforward	1,117	769
Future lease payments	38	37
Derivative activity	9	0
Capital loss carryover	27	0
Other	24	18
Total deferred tax assets	1,579	879
Valuation allowance	(1,539)	(87)	$ (609)
Net deferred tax asset	$ 0	$ 407