Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,321	$ 3,951	$ 6,634	$ 8,001
Interest cost	6,444	7,142	12,878	14,291
Expected return on plan assets	(14,583)	(15,698)	(29,166)	(31,353)
Amortization of prior service cost (credit)	151	268	302	514
Amortization of actuarial losses (gains)	2,084	1,833	4,167	3,666
Special termination benefits	2,900	0	2,900	0
Curtailment loss	180	0	180	0
Net periodic cost (credit)	497	(2,504)	(2,105)	(4,881)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	69	136	136	279
Interest cost	253	349	507	709
Amortization of prior service cost (credit)	(850)	(736)	(1,700)	(1,472)
Amortization of actuarial losses (gains)	(170)	(63)	(288)	(127)
Net periodic cost (credit)	$ (698)	$ (314)	$ (1,345)	$ (611)