Pension and Postretirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,684	$ 6,548	$ 15,832	$ 13,402
Interest cost	8,252	8,673	16,613	18,030
Expected return on assets	(12,403)	(10,071)	(23,032)	(20,613)
Prior service cost (credit)	26	113	52	155
Actuarial loss (gain)	3,117	3,551	6,413	7,087
Net periodic benefit cost (credit)	6,676	8,814	15,878	18,061
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	16	22	38	40
Interest cost	134	198	259	365
Prior service cost (credit)	(426)	(471)	(1,037)	(871)
Actuarial loss (gain)	(48)	(99)	(105)	(182)
Net periodic benefit cost (credit)	$ (324)	$ (350)	$ (845)	$ (648)