Pension Plans and Other Postretirement Benefits - Net Periodic Defined Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,087	$ 5,103
Interest cost on projected benefit obligation	20,200	18,805
Expected return on plan assets	(30,600)	(30,875)
Amortization of prior service cost (credit)	125	125
Amortization of net actuarial loss (gain)	4,450	11,100
Net periodic cost (benefit)	(1,738)	4,258
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	200	226
Interest cost on projected benefit obligation	603	557
Amortization of prior service cost (credit)	(1,175)	(1,175)
Amortization of net actuarial loss (gain)	(300)	(200)
Net periodic cost (benefit)	$ (672)	$ (592)