Other Accrued Expenses Other Accrued Expenses (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule of Other Liabilities [Line Items]
Saddle Butte Rockies Midstream Amendment Payment	$ 24,100
Production Tax Liability	63,454		$ 50,476
Other Accrued Liabilities, Noncurrent	94,557		57,333
Accrued Employee Benefits, Current	10,901		22,383
Asset Retirement Obligation, Current	15,944		15,801
Accrued Environmental Loss Contingencies, Current	2,074		1,374
Gas Gathering, Transportation, Marketing and Processing Costs	(7,313)	$ (5,902)
Amortization of Other Deferred Charges	400
Non Current Liabilities
Schedule of Other Liabilities [Line Items]
Gas Gathering, Transportation, Marketing and Processing Costs	21,608		0
Other Accrued Liabilities	9,495		6,857
Current Liabilities
Schedule of Other Liabilities [Line Items]
Gas Gathering, Transportation, Marketing and Processing Costs	2,010		0
Other Accrued Liabilities	$ 2,848		$ 3,429