Pension Plans and Other Postretirement Benefits - Net Periodic Defined Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,103	$ 4,908
Interest cost on projected benefit obligation	18,805	19,691
Expected return on plan assets	(30,875)	(27,200)
Amortization of prior service cost (credit)	125	125
Amortization of net actuarial loss	11,100	6,800
Net periodic benefit cost	4,258	4,324
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	226	383
Interest cost on projected benefit obligation	557	1,080
Amortization of prior service cost (credit)	(1,175)	(350)
Amortization of net actuarial loss	(200)	(25)
Net periodic benefit cost	$ (592)	$ 1,088