Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Full-cost ceiling impairment	$ 0	$ 158,633	$ 801,166
Increase in valuation allowance	101,917	(33,688)	(145,777)
Provisional income tax	51,500
Net current income tax (benefit) provision	8,157	$ 1,036	$ (2,959)
Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	498,400
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 17,000