Recent Accounting Pronouncements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Net periodic cost (credit)	$ 211,000	$ 268,000	$ 633,000	$ 898,000	$ 865,000
OPEB
Defined Benefit Plan Disclosure [Line Items]
Net periodic cost (credit)	$ (34,000)	$ (149,000)	$ (102,000)	$ (448,000)	$ (597,000)