Employee Benefits - Components of Net Periodic Pension Cost and Other Changes in Plan Assets and Benefit (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Components of Net Periodic Pension Cost
Service cost—benefits earned during the period	$ 606	$ 516	$ 1,951	$ 1,778
Interest cost on projected benefit obligation	1,120	1,141	3,319	3,353
Expected return on plan assets	(1,772)	(1,710)	(5,385)	(5,180)
Amortization of prior service (credit) cost	(34)	(34)	(104)	(104)
Recognized net actuarial loss	586	287	1,521	705
Net Periodic Pension Expense	$ 506	$ 200	$ 1,302	$ 552