Employee Benefit Plans - Net Periodic Pension Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 666	$ 569	$ 1,324	$ 1,131
Interest cost	1,308	1,279	2,603	2,544
Expected return on plan assets	(2,551)	(2,049)	(5,073)	(4,076)
Recognized net actuarial loss	1,162	1,100	2,311	2,187
Net periodic pension (benefit) cost	$ 585	$ 899	$ 1,165	$ 1,786
Weighted-Average Assumptions:
Discount rate	3.83%	4.49%	3.83%	4.49%
Expected return on assets	7.00%	7.00%	7.00%	7.00%
Rate of compensation increase	3.00%	3.00%	3.00%	3.00%
Prior to Age 45 [Member]
Weighted-Average Assumptions:
Rate of compensation increase	3.50%	3.50%	3.50%	3.50%