Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 25,037	$ 24,372	$ 20,520
Interest cost on benefit obligation	83,410	72,731	69,162
Expected return on plan assets	(108,473)	(96,155)	(91,568)
Amortization of prior service credit	(3,228)	(6,005)	(6,552)
Recognized net actuarial loss	30,145	44,825	14,494
Net periodic benefit costs	26,891	39,768	6,056
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,595	914	605
Interest cost on benefit obligation	4,971	2,995	2,778
Amortization of prior service credit	(1,359)	(1,359)	(1,359)
Recognized net actuarial loss	60	106
Net periodic benefit costs	$ 5,267	$ 2,656	$ 2,024