Schedule of Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,752	$ 4,996	$ 17,257	$ 12,129
Interest cost	8,287	7,979	24,863	20,952
Expected return on assets	(9,095)	(8,627)	(27,285)	(24,030)
Prior service cost (credit)	106	111	317	334
Actuarial loss (gain)	3,223	1,011	13,923	3,034
Special termination benefit	382	13,680	1,844	13,680
Net periodic benefit cost (credit)	8,655	19,150	30,919	26,099
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	34	57	103	150
Interest cost	232	302	696	861
Prior service cost (credit)	(574)	(814)	(1,724)	(2,441)
Actuarial loss (gain)	(77)	(66)	(232)	(199)
Net periodic benefit cost (credit)	$ (385)	$ (521)	$ (1,157)	$ (1,629)