Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 20,346	$ 20,193	$ 25,037
Interest cost on benefit obligation	74,704	79,270	83,410
Expected return on plan assets	(123,127)	(108,524)	(108,473)
Amortization of prior service cost (credit)	557	557	(3,228)
Recognized net actuarial (gain) loss	43,793	29,263	30,145
Net periodic benefit cost	16,273	20,759	26,891
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	938	1,172	1,595
Interest cost on benefit obligation	2,293	3,716	4,971
Amortization of prior service cost (credit)	(4,729)	(1,359)	(1,359)
Recognized net actuarial (gain) loss	(826)	(988)	60
Net periodic benefit cost	$ (2,324)	$ 2,541	$ 5,267