Employee Benefits - Components of Net Periodic Benefit Cost and Changes in Plan Assets and Benefit Obligations Recognized in OCI (Detail) (Postretirement Benefit Plan [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 1,295	$ 1,526	$ 1,357
Interest cost on projected benefit obligation	4,896	4,503	5,656
Net amortization of deferrals	(4,109)	(3,040)	(1,770)
Net periodic benefit cost	2,082	2,989	5,243
Net actuarial loss (gain)	7,706	(15,524)
Recognized actuarial loss	(540)	(1,672)
Recognized prior service credit	4,649	4,712
Total recognized in OCI (before tax effects)	$ 11,815	$ (12,484)