Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,532	$ 2,950	$ 9,967	$ 9,584
Interest cost	6,482	6,473	19,114	19,351
Expected return on plan assets	(15,832)	(14,653)	(47,258)	(43,819)
Amortization of prior service cost (credit)	75	113	235	415
Amortization of actuarial losses (gains)	1,501	2,326	5,208	6,493
Special termination benefits	0	0	0	2,900
Curtailment loss	0	0	0	180
Net periodic benefit credit	(4,242)	(2,791)	(12,734)	(4,896)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	9	52	79	188
Interest cost	168	230	520	737
Amortization of prior service cost (credit)	(857)	(816)	(2,564)	(2,516)
Amortization of actuarial losses (gains)	(172)	(187)	(447)	(475)
Net periodic benefit credit	$ (852)	$ (721)	$ (2,412)	$ (2,066)