Pension Plans and Other Postretirement Benefits - Net Periodic Defined Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,086	$ 5,048	$ 15,258	$ 15,145
Interest cost on projected benefit obligation	18,676	19,818	56,029	59,452
Expected return on plan assets	(30,781)	(27,131)	(92,344)	(81,393)
Amortization of prior service cost (credit)	139	139	417	418
Amortization of net actuarial loss (gain)	10,948	7,316	32,844	21,947
Net periodic benefit cost	4,068	5,190	12,204	15,569
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	234	293	703	878
Interest cost on projected benefit obligation	573	929	1,719	2,787
Amortization of prior service cost (credit)	(1,182)	(340)	(3,546)	(1,019)
Amortization of net actuarial loss (gain)	(206)	(247)	(619)	(741)
Net periodic benefit cost	$ (581)	$ 635	$ (1,743)	$ 1,905