Schedule of 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
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,776	$ 5,513	$ 11,082	$ 11,505
Interest cost	9,331	8,213	17,938	16,575
Expected return on assets	(9,822)	(8,887)	(18,848)	(18,190)
Prior service cost (credit)	91	101	175	211
Actuarial loss (gain)	3,146	7,351	6,036	10,700
Settlement charge			59
Special termination benefit	638	1,206	764	1,462
Net periodic benefit cost (credit)	9,160	13,497	17,206	22,263
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	24	34	43	68
Interest cost	259	229	432	464
Prior service cost (credit)	(609)	(570)	(981)	(1,150)
Actuarial loss (gain)	(139)	(77)	(249)	(154)
Special termination benefit	(8)		(8)
Net periodic benefit cost (credit)	$ (473)	$ (384)	$ (763)	$ (772)