Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,313	$ 4,050
Interest cost	6,434	7,149
Expected return on plan assets	(14,583)	(15,655)
Amortization of prior service cost (credit)	151	246
Amortization of actuarial losses (gains)	2,083	1,833
Net periodic benefit credit	(2,602)	(2,377)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	67	143
Interest cost	254	360
Amortization of prior service cost (credit)	(850)	(736)
Amortization of actuarial losses (gains)	(118)	(64)
Net periodic benefit credit	$ (647)	$ (297)