Income Taxes - Temporary Differences Related to Deferred Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred income tax assets:
Post-retirement liability	$ 770	$ 910
Deferred income	393	543
Share-based compensation	1,581	1,158
Capital loss carryforwards	379	0
State tax credit carryforwards	3,245	3,488
State operating loss carryforwards	1,505	1,434
Inventories	1,476	1,346
Other	1,231	766
Gross deferred income tax assets	10,580	9,645
Less: valuation allowance	(1,452)	(148)
Net deferred income tax assets	9,128	9,497
Deferred income tax liabilities:
Fixed assets	(10,497)	(9,255)
Other	(308)	(254)
Gross deferred income tax liabilities	(10,805)	(9,509)
Net deferred income tax liability	$ (1,677)	$ (12)