Income Taxes - Temporary Differences Related to Deferred Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets:
Post-retirement liability	$ 910	$ 1,621
Deferred income	543	1,176
Share-based compensation	1,158	1,313
Capital loss carryforwards	0	716
State tax credit carryforwards	3,488	3,204
State operating loss carryforwards	1,434	1,151
Inventories	1,346	2,560
Other	766	1,381
Gross deferred income tax assets	9,645	13,122
Less: valuation allowance	(148)	(726)
Net deferred income tax assets	9,497	12,396
Deferred income tax liabilities:
Fixed assets	(9,255)	(14,313)
Equity method investments	0	(969)
Other	(254)	(546)
Gross deferred income tax liabilities	(9,509)	(15,828)
Net deferred income tax liability	$ (12)	$ (3,432)