Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,542	$ 5,752	$ 16,624	$ 17,257
Interest cost	8,970	8,287	26,908	24,863
Expected return on assets	(9,425)	(9,095)	(28,272)	(27,285)
Prior service cost (credit)	87	106	262	317
Actuarial loss (gain)	3,018	3,223	9,055	13,923
Settlement charge			59
Special termination benefit		382	764	1,844
Net periodic benefit cost (credit)	8,192	8,655	25,400	30,919
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	34	65	103
Interest cost	216	232	648	696
Prior service cost (credit)	(491)	(574)	(1,471)	(1,724)
Actuarial loss (gain)	(125)	(77)	(374)	(232)
Special termination benefit			(8)
Net periodic benefit cost (credit)	$ (378)	$ (385)	$ (1,140)	$ (1,157)