Pension Plans and Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,555	$ 3,243	$ 9,329	$ 9,724
Interest cost	12,253	12,765	36,017	38,369
Expected return on plan assets	(16,631)	(15,861)	(49,986)	(47,571)
Amortization of prior service cost (credit)	99	125	299	375
Net periodic benefit cost (benefit)	(1,724)	272	(4,341)	897
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	249	240	748	743
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	0	0	0	0
Net periodic benefit cost (benefit)	$ 249	$ 240	$ 748	$ 743