Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,950	$ 3,688	$ 9,584	$ 11,689
Interest cost	6,473	6,753	19,351	21,044
Expected return on plan assets	(14,653)	(15,692)	(43,819)	(47,045)
Amortization of prior service cost (credit)	113	264	415	778
Amortization of actuarial losses (gains)	2,326	627	6,493	4,293
Special termination benefits	0	0	2,900	0
Curtailment (gain) loss	0	(482)	180	(482)
Net periodic benefit credit	(2,791)	(4,842)	(4,896)	(9,723)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	52	56	188	335
Interest cost	230	256	737	965
Amortization of prior service cost (credit)	(816)	(817)	(2,516)	(2,289)
Amortization of actuarial losses (gains)	(187)	(133)	(475)	(260)
Net periodic benefit credit	$ (721)	$ (638)	$ (2,066)	$ (1,249)