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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,069	$ 5,189	$ 10,172	$ 10,097
Interest cost on projected benefit obligation	18,548	19,943	37,353	39,634
Expected return on plan assets	(30,688)	(27,062)	(61,563)	(54,262)
Amortization of prior service cost (credit)	153	154	278	279
Amortization of net actuarial loss (gain)	10,796	7,831	21,896	14,631
Net periodic benefit cost	3,878	6,055	8,136	10,379
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	243	202	469	585
Interest cost on projected benefit obligation	589	778	1,146	1,858
Amortization of prior service cost (credit)	(1,189)	(329)	(2,364)	(679)
Amortization of net actuarial loss (gain)	(213)	(469)	(413)	(494)
Net periodic benefit cost	$ (570)	$ 182	$ (1,162)	$ 1,270