Stockholder's Equity - Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Roll Forward]
Beginning balance	$ (4,272)	$ (4,878)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(1,440)	828
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	901	(1,194)
Net current-period other comprehensive income (loss)	(539)	(366)
Ending balance	(5,718)	(5,244)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	(907)
UnrealizedGainsLossesFromDefinedBenefitPensionAndPostretirementPlanItems [Member]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Roll Forward]
Beginning balance	(4,743)	(5,360)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	(9)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	189	180
Net current-period other comprehensive income (loss)	189	171
Ending balance	(5,576)	(5,189)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	(1,022)
Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Roll Forward]
Beginning balance	471	482
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(1,440)	837
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	712	(1,374)
Net current-period other comprehensive income (loss)	(728)	(537)
Ending balance	(142)	$ (55)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	$ 115