Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Payroll, bonus and vacation expense	$ 2,299	$ 2,633
Non income taxes	3,548	2,738
Royalty expenses	2,222	2,410
Accrued interest	86	132
Health claims	680	871
Workers' compensation & pneumoconiosis	1,750	1,750
Deferred revenues	985
Other	963	652
Total	$ 12,533	$ 11,186