Income Taxes - Reconciliation of taxes computed at the corporate federal income tax rate to the reported income tax (provision) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Pre-tax net book income (1)	$ (3,427)	$ 4,434	$ (2,045)
Statutory federal income tax (provision) benefit	1,165	(1,552)	716
State income tax (provision) benefit	736	(387)	(233)
Federal statutory depletion	153	62	144
Federal tax rate adjustment	2,007	0	0
Valuation allowance	(1,038)	0	0
Other	(78)	(44)	143
Total provision for (benefit from) income taxes (1)	$ 2,945	$ (1,921)	$ 770
Effective income tax rate	86.00%	43.00%	38.00%
Pre-tax losses from investments in unconsolidated affiliates		$ 2,200