Pension Plans and Other Postretirement Benefits - Net Periodic Defined Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,908	$ 6,382
Interest cost on projected benefit obligation	19,691	20,883
Expected return on plan assets	(27,200)	(27,814)
Amortization of prior service cost (credit)	125	(825)
Amortization of net actuarial loss	6,800	8,300
Net periodic benefit cost	4,324	6,926
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	383	458
Interest cost on projected benefit obligation	1,080	1,205
Amortization of prior service cost (credit)	(350)	(350)
Amortization of net actuarial loss	(25)
Net periodic benefit cost	$ 1,088	$ 1,313