Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 8,148	$ 6,572
Interest cost	8,361	9,008
Expected return on assets	(10,629)	(9,936)
Prior service cost (credit)	26	78
Actuarial loss (gain)	3,296	3,524
Net periodic benefit cost (credit)	9,202	9,246
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	24
Interest cost	125	185
Prior service cost (credit)	(611)	(435)
Actuarial loss (gain)	(57)	(72)
Net periodic benefit cost (credit)	$ (521)	$ (298)