Pension and Postretirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,872	$ 8,148
Interest cost	9,395	8,361
Expected return on assets	(11,966)	(10,629)
Prior service cost (credit)	2	26
Actuarial loss (gain)	3,932	3,296
Net periodic benefit cost (credit)	9,235	9,202
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	15	22
Interest cost	131	125
Prior service cost (credit)	(190)	(611)
Actuarial loss (gain)	(118)	(57)
Net periodic benefit cost (credit)	$ (162)	$ (521)