Pension Plans and Other Postretirement Benefits - Net Periodic Pension Expense for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,382	$ 6,000
Interest cost on projected benefit obligation	20,883	17,775
Expected return on plan assets	(27,814)	(23,575)
Amortization of prior service credit	(825)	(1,525)
Amortization of net actuarial loss	8,300	11,175
Net periodic benefit cost	6,926	9,850
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	458	200
Interest cost on projected benefit obligation	1,205	650
Amortization of prior service credit	(350)	(350)
Amortization of net actuarial loss		25
Net periodic benefit cost	$ 1,313	$ 525