Pension and Postretirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,911	$ 7,457	$ 23,732	$ 20,060
Interest cost	8,307	9,729	24,921	27,074
Expected return on assets	(11,516)	(10,691)	(34,549)	(29,818)
Prior service cost (credit)	27	34	77	237
Actuarial loss (gain)	3,206	3,604	9,621	10,643
Settlement charge	2,692		2,692
Net periodic benefit cost (credit)	10,627	10,133	26,494	28,196
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	19	20	58	60
Interest cost	130	182	389	547
Prior service cost (credit)	(519)	(436)	(1,556)	(1,307)
Actuarial loss (gain)	(53)	(91)	(158)	(273)
Net periodic benefit cost (credit)	$ (423)	$ (325)	$ (1,267)	$ (973)