Divestitures and Assets Held for Sale (Details) $ in Thousands		3 Months Ended		6 Months Ended			12 Months Ended
	Mar. 26, 2018 a	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)		Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Properties held for sale, net		$ 0		$ 0			$ 5,280
Net divestiture proceeds				12,520	[1]	$ 742,215
Net gain on divestiture activity		$ 262	$ 39,501	$ 323		424,870
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | PRB Divestiture 2018 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net acreage divested | a	112,000
Net divestiture proceeds						490,800	492,200
Net gain on divestiture activity						410,100	410,600
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Divide County Divestiture and Halff East Divestiture 2018 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net divestiture proceeds						250,800	252,200
Net gain on divestiture activity						$ 15,700	$ 15,400

[1] ⁽¹⁾  As of June 30, 2019, a portion of net proceeds from the sale of oil and gas properties was restricted for future property acquisitions. Restricted cash is included in the other noncurrent assets line item on the accompanying unaudited condensed consolidated balance sheets (“accompanying balance sheets”).