Income Taxes (Schedule Of Deferred Tax Assets And Deferred Tax Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Valuation Allowance [Line Items]
Net operating loss carryforward	$ 1,111	$ 1,039
Deferred Tax Assets, Tax Credit Carryforwards, Research	110	101
Asset retirement obligations	61	41
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost	29	40
Deferred Tax Assets, Hedging Transactions	68	0
South Texas Divestiture	62	75
Lease deferred tax assets	167	191
Other	47	47
Total deferred tax assets	1,655	1,534
Oil and gas properties, principally due to differences in basis, depletion and the deduction of intangible drilling costs for tax purposes	(2,741)	(2,628)
Other property and equipment, principally due to the deduction of bonus depreciation for tax purposes	(196)	(189)
Net deferred hedge gains	0	(9)
South Texas Divestiture	(16)	(35)
Lease deferred tax liabilities	(43)	(61)
Deferred Tax Liability, Convertible Notes	(23)	0
Other	(2)	(5)
Net deferred tax liability	3,021	2,927
Deferred Income Tax Liabilities, Net	1,366	1,393
Unrecognized tax benefits	0	$ 39	$ 141	$ 124
Research Tax Credit Carryforward [Member] | U.S.
Valuation Allowance [Line Items]
Deferred Tax Assets, Tax Credit Carryforwards, Research	110
Domestic Tax Authority [Member]
Valuation Allowance [Line Items]
Net operating loss carryforward	5,000
COLORADO
Valuation Allowance [Line Items]
Net operating loss carryforward	$ 177