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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,189	$ 6,137	$ 10,097	$ 12,519
Interest cost on projected benefit obligation	19,943	20,822	39,634	41,705
Expected return on plan assets	(27,062)	(26,423)	(54,262)	(54,237)
Amortization of prior service cost (credit)	154	(789)	279	(1,614)
Amortization of net actuarial loss (gain)	7,831	6,773	14,631	15,073
Net periodic benefit cost	6,055	6,520	10,379	13,446
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	202	340	585	798
Interest cost on projected benefit obligation	778	1,281	1,858	2,486
Amortization of prior service cost (credit)	(329)	(330)	(679)	(680)
Amortization of net actuarial loss (gain)	(469)	30	(494)	30
Net periodic benefit cost	$ 182	$ 1,321	$ 1,270	$ 2,634