Income Taxes (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Effective tax rate		(15.00%)	(86.00%)	0.00%
Corporate income tax rate		21.00%
Alternative minimum tax carryforward, expected refund		$ 30,000,000
Sequestered amount relating to alternative minimum tax refunds		2,000,000
Income tax refund received		32,000,000	$ 1,000,000
Income taxes paid				$ 6,300,000
Operating loss carryforwards subject to a section 382 limitation		90,000,000
Discrete tax expense	$ 408,000,000
Operating loss carryforward valuation allowance	$ 87,000,000
Release of valuation allowance		0	(522,000,000)
Change in valuation allowance		1,034,000,000	$ (522,000,000)	$ (121,000,000)
Unrecognized tax benefits that would impact effective tax rate		0
Montage Resources Corporation
Income Taxes [Line Items]
Net operating loss carryforward		1,000,000,000
Operating loss carryforwards subject to a section 382 limitation		1,700,000
Statutory depletion carryforward
Income Taxes [Line Items]
Tax credit carryforward		13,000,000
Interest deduction carryforward
Income Taxes [Line Items]
Tax credit carryforward		55,000,000
Exploration Program in Canada
Income Taxes [Line Items]
Net operating loss carryforward		29,000,000
Federal
Income Taxes [Line Items]
Net operating loss carryforward		4,500,000,000
Statel
Income Taxes [Line Items]
Net operating loss carryforward		$ 2,000,000,000