Schedule of Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,507	$ 6,290
Interest cost	8,871	8,112
Expected return on assets	(9,434)	(9,104)
Prior service cost (credit)	88	105
Actuarial loss (gain)	2,831	3,108
Settlement charge	59
Special termination benefit	126	256
Net periodic benefit cost (credit)	8,048	8,767
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	34	39
Interest cost	238	230
Prior service cost (credit)	(462)	(576)
Actuarial loss (gain)	(99)	(82)
Net periodic benefit cost (credit)	$ (289)	$ (389)