Pension Plan - 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
Retirement Benefits [Abstract]
Contributions to the plan			$ 0		$ 500,000
Net periodic benefit costs, total	$ 84,000	$ 82,000	$ 253,000	$ 247,000