Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) (Postretirement Benefit Plan [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 326	$ 362	$ 971	$ 1,086
Interest cost on projected benefit obligation	1,197	1,168	3,672	3,504
Net amortization and deferrals	1,124	663	3,082	1,989
Net periodic benefit cost	$ 399	$ 867	$ 1,561	$ 2,601