Defined Benefit Pension Plan - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Postemployment Benefits [Abstract]
Service cost	$ 26	$ 24	$ 54	$ 46
Interest cost	26	27	53	52
Amortization	23	22	47	44
Net periodic benefit cost	$ 75	$ 73	$ 154	$ 142