Other Accrued Expenses Other Accrued Expenses (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Schedule of Other Liabilities [Line Items]
Amortization of Other Deferred Charges	$ 500	$ 400
Production Tax Liability	78,805		$ 61,310
Operating and Finance Lease Liability, Noncurrent	20,063		0
Other Accrued Liabilities, Noncurrent	$ 125,063		92,664
Document Period End Date	Mar. 31, 2019
Non Current Liabilities
Schedule of Other Liabilities [Line Items]
Increase (Decrease) in Accrued Cost of Oil and Gas Reclamation	$ 22,207		22,710
Other Accrued Liabilities	3,988		8,644
Current Liabilities
Schedule of Other Liabilities [Line Items]
Other Accrued Liabilities	$ 19,651		$ 20,686