Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,267	$ 3,321	$ 6,435	$ 6,634
Interest cost	6,362	6,444	12,632	12,878
Expected return on plan assets	(15,713)	(14,583)	(31,426)	(29,166)
Amortization of prior service cost (credit)	80	151	160	302
Amortization of actuarial losses (gains)	1,854	2,084	3,707	4,167
Special termination benefits	0	2,900	0	2,900
Curtailment loss	0	180	0	180
Net periodic benefit (credit) cost	(4,150)	497	(8,492)	(2,105)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	34	69	70	136
Interest cost	176	253	352	507
Amortization of prior service cost (credit)	(854)	(850)	(1,707)	(1,700)
Amortization of actuarial losses (gains)	(136)	(170)	(275)	(288)
Net periodic benefit (credit) cost	$ (780)	$ (698)	$ (1,560)	$ (1,345)