Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 24,372	$ 20,520	$ 24,360
Interest cost on benefit obligation	72,731	69,162	60,130
Expected return on plan assets	(96,155)	(91,568)	(87,353)
Amortization of prior service credit	(6,005)	(6,552)	(6,556)
Recognized net actuarial loss	44,825	14,494	41,076
Net periodic benefit costs	39,768	6,056	31,657
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	914	605	742
Interest cost on benefit obligation	2,995	2,778	2,691
Amortization of prior service credit	(1,359)	(1,359)	(1,359)
Recognized net actuarial loss	106		360
Net periodic benefit costs	$ 2,656	$ 2,024	$ 2,434