Income Taxes - Temporary Differences Related to Deferred Income Taxes (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred income tax assets:
Post-retirement liability	$ 1,621	$ 1,848
Deferred income	1,176	1,343
Share-based compensation	1,313	2,247
Capital loss carryforwards	716	1,444
State tax credit carryforwards	3,204	2,653
State operating loss carryforwards	1,151	2,216
Inventories	2,560	1,684
Other	1,381	2,224
Gross deferred income tax assets	13,122	15,659
Less: valuation allowance	(726)	(1,444)
Net deferred income tax assets	12,396	14,215
Deferred income tax liabilities:
Fixed assets	(14,313)	(16,050)
Equity method investments	(969)	0
Other	(546)	(922)
Gross deferred income tax liabilities	(15,828)	(16,972)
Net deferred income tax liability	$ (3,432)	$ (2,757)