Pension Plan - Components of 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, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost - benefits earned during year	$ 707	$ 643	$ 1,406	$ 1,279
Interest cost on projected benefit obligation	1,228	1,096	2,442	2,180
Expected return on plan assets	(2,390)	(1,907)	(4,753)	(3,793)
Amortization of prior service cost	7	6	13	12
Amortization of net loss	758	803	1,508	1,597
Net periodic pension cost	$ 310	$ 641	$ 616	$ 1,275