Recent Accounting Pronouncements Not Yet Adopted - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Net periodic cost (credit)	$ 211,000	$ 342,000	$ 422,000	$ 630,000	$ 865,000
OPEB
Defined Benefit Plan Disclosure [Line Items]
Net periodic cost (credit)	$ (34,000)	$ (150,000)	$ (68,000)	$ (299,000)	$ (597,000)