Pension Plan - Components of Net Periodic Pension Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost - benefits earned during year	$ 703	$ 846	$ 2,095	$ 2,509
Interest cost on projected benefit obligation	1,280	1,228	3,813	3,643
Expected return on plan assets	(1,940)	(1,950)	(5,778)	(5,785)
Amortization of prior service cost	8	7	20	19
Amortization of net loss	759	801	2,261	2,378
Net periodic pension cost	$ 810	$ 932	$ 2,411	$ 2,764