Employee Benefit Plans - Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee Benefit Plan [Line Items]
Net actuarial loss	$ (871)	$ (1,871)
Tax effect	183	654
Other comprehensive income (loss)	(690)	(1,217)	$ (1,153)	$ (1,063)
Employee Benefit Plan [Member]
Employee Benefit Plan [Line Items]
Other comprehensive income (loss)	$ (688)	$ (1,217)