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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,617	$ 7,684	$ 15,422	$ 15,832
Interest cost	9,300	8,252	18,856	16,613
Expected return on assets	(11,824)	(12,403)	(23,930)	(23,032)
Prior service cost (credit)	1	26	3	52
Actuarial loss (gain)	3,941	3,117	7,918	6,413
Net periodic benefit cost (credit)	9,035	6,676	18,269	15,878
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	14	16	48	38
Interest cost	125	134	281	259
Prior service cost (credit)	(185)	(426)	(399)	(1,037)
Actuarial loss (gain)	(109)	(48)	(248)	(105)
Net periodic benefit cost (credit)	$ (155)	$ (324)	$ (318)	$ (845)