Employee Benefit Plans - Components of and Weighted-Average Assumptions Used in Determining Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of net periodic benefit cost:
Service cost - benefits earned during year	$ 2,799	$ 3,355	$ 2,909
Interest cost on projected benefit obligation	5,094	4,870	4,745
Expected return on plan assets	(7,719)	(7,735)	(7,229)
Amortization of prior service cost	26	26	45
Amortization of net loss	3,020	3,179	1,471
Net periodic pension cost	3,220	3,695	1,941
Other changes in plan assets and benefit obligations recognized in other comprehensive income:
Net (gain) loss for period	3,329	(5,106)	24,934
Amortization of prior service cost	(26)	(26)	(45)
Amortization of net loss	(3,020)	(3,179)	(1,471)
Total recognized in other comprehensive income	283	(8,311)	23,418
Total recognized in net periodic pension cost and other comprehensive income	$ 3,503	$ (4,616)	$ 25,359
Weighted-average assumptions used to determine net periodic pension cost:
Discount rate	4.74%	4.33%	5.17%
Rate of compensation increase	3.82%	3.77%	3.97%
Expected long-term return on assets	6.79%	7.00%	7.25%