Acquisitions and Divestitures (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended					6 Months Ended		8 Months Ended		12 Months Ended
	Feb. 28, 2018	Jan. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Jul. 31, 2018	Sep. 30, 2018	Dec. 11, 2017	Nov. 20, 2017
Business Acquisition [Line Items]
Acquisition of oil and gas properties								$ 8,222	$ 0
Proceeds from divestitures of oil and gas properties, net								$ 6,034	$ 345,789
Devon Acquisition
Business Acquisition [Line Items]
Agreed upon Purchase Price of Oil and Gas Property and Equipment					$ 215,000
Deposit For Acquisition Of Oil And Gas Properties					21,500
Payments to Acquire Oil and Gas Property				$ 183,400
Acquisition of oil and gas properties			$ (8,300)
Estimated Aggregate Purchase Price										$ 196,600
Eagle Ford Shale Divestiture
Business Acquisition [Line Items]
Sale Price of Oil and Gas Property and Equipment													$ 245,000
Proceeds from divestitures of oil and gas properties, net											$ 245,700
Cash Paid for Post-Closing Adjustments to Divestitures					(500)
Niobrara Divestiture
Business Acquisition [Line Items]
Sale Price of Oil and Gas Property and Equipment														$ 140,000
Proceeds from divestitures of oil and gas properties, net						$ 122,600	$ 14,000					$ 135,600
Cash Paid for Post-Closing Adjustments to Divestitures					$ (1,000)
Deposit Received Prior To Closing [Member] | Eagle Ford Shale Divestiture
Business Acquisition [Line Items]
Proceeds from divestitures of oil and gas properties, net							$ 24,500
Cash Received At Closing [Member] | Eagle Ford Shale Divestiture
Business Acquisition [Line Items]
Proceeds from divestitures of oil and gas properties, net		$ 211,700
Cash Received Post Closing [Member] | Eagle Ford Shale Divestiture
Business Acquisition [Line Items]
Proceeds from divestitures of oil and gas properties, net	$ 10,000