Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,572	$ 5,507
Interest cost	9,008	8,871
Expected return on assets	(9,936)	(9,434)
Prior service cost (credit)	78	88
Actuarial loss (gain)	3,524	2,831
Settlement charge		59
Special termination benefit		126
Net periodic benefit cost (credit)	9,246	8,048
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	24	34
Interest cost	185	238
Prior service cost (credit)	(435)	(462)
Actuarial loss (gain)	(72)	(99)
Net periodic benefit cost (credit)	$ (298)	$ (289)