Income Taxes - Temporary Differences Related to Deferred Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred income tax assets:
Post-retirement liability	$ 717	$ 770
Deferred income	300	393
Share-based compensation	1,238	1,581
Capital loss carryforwards	91	379
State tax credit carryforwards	3,198	3,245
State operating loss carryforwards	1,529	1,505
Inventories	1,738	1,476
Operating lease liabilities	1,582	0
Other	1,291	1,231
Gross deferred income tax assets	11,684	10,580
Less: valuation allowance	(1,284)	(1,452)
Net deferred income tax assets	10,400	9,128
Deferred income tax liabilities:
Fixed assets	(10,332)	(10,497)
Operating lease right-of-use assets	(1,577)	0
Other	(420)	(308)
Gross deferred income tax liabilities	(12,329)	(10,805)
Net deferred income tax liability	$ (1,929)	$ (1,677)