Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 20,193	$ 25,037	$ 24,372
Interest cost on benefit obligation	79,270	83,410	72,731
Expected return on plan assets	(108,524)	(108,473)	(96,155)
Amortization of prior service cost (credit)	557	(3,228)	(6,005)
Recognized net actuarial loss	29,263	30,145	44,825
Net periodic benefit costs	20,759	26,891	39,768
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,172	1,595	914
Interest cost on benefit obligation	3,716	4,971	2,995
Amortization of prior service cost (credit)	(1,359)	(1,359)	(1,359)
Recognized net actuarial loss	(988)	60	106
Net periodic benefit costs	$ 2,541	$ 5,267	$ 2,656