Income Taxes - Income Tax Expense (Benefit) Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Amount computed using the statutory rate	$ (2,655)	$ (31,623)	$ (102,257)
Impact of rate change on deferred tax	64,915	0	0
State & local taxes	(368)	(2,000)	(6,477)
Percentage depletion carryforward	(66)	(163)	(404)
Non-deductible stock option expense	305	77	90
Share-based compensation	64	707	1,317
Other	(21)	1,415	113
Change in valuation allowance	(63,123)	32,130	110,244
Income tax expense (benefit)	$ (949)	$ 543	$ 2,626