Employee Benefit Plans - Components of Net Periodic Pension Cost for Defined Benefit Pension Plan and Retirement Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Service cost	$ 531	$ 531	$ 785
Interest cost	1,329	1,367	1,350
Expected return on plan assets	(1,530)	(1,482)	(1,495)
Amortization of actuarial losses	810	762	887
Net periodic pension cost	$ 1,140	$ 1,178	$ 1,527