Recent Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Net periodic cost (credit)	$ 1,093	$ 865	$ 363
OPEB
Defined Benefit Plan Disclosure [Line Items]
Net periodic cost (credit)	$ (138)	$ (597)	$ (614)