Pension Plans and Other Postretirement Benefits - Net Periodic Defined Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,559	$ 5,069	$ 8,646	$ 10,172
Interest cost on projected benefit obligation	20,590	18,548	40,790	37,353
Expected return on plan assets	(30,470)	(30,688)	(61,070)	(61,563)
Amortization of prior service cost (credit)	154	153	279	278
Amortization of net actuarial loss (gain)	6,546	10,796	10,996	21,896
Net periodic cost (benefit)	1,379	3,878	(359)	8,136
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	229	243	429	469
Interest cost on projected benefit obligation	569	589	1,172	1,146
Amortization of prior service cost (credit)	(1,190)	(1,189)	(2,365)	(2,364)
Amortization of net actuarial loss (gain)	(323)	(213)	(623)	(413)
Net periodic cost (benefit)	$ (715)	$ (570)	$ (1,387)	$ (1,162)