Income Taxes - Temporary Differences Related to Deferred Income Taxes (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred income tax assets:
Post-retirement liability	$ 1,848	$ 1,968
Deferred income	1,343	1,637
Stock based compensation	2,247	2,108
Federal operating loss carryforwards	0	5,029
Capital loss carryforward	1,444	1,311
State tax credits	2,653	2,423
State operating loss carryforwards	2,216	4,574
Inventories	1,684	514
Other	2,224	2,891
Gross deferred income tax assets	15,659	22,455
Less: valuation allowance	(1,444)	(3,829)
Net deferred income tax assets	14,215	18,626
Deferred income tax liabilities:
Fixed assets	(16,050)	(18,823)
Other	(922)	(1,176)
Gross deferred income tax liabilities	(16,972)	(19,999)
Net deferred income tax liability	$ (2,757)	$ (1,373)