Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,000	$ 5,100
Interest cost on projected benefit obligation	17,775	17,250
Expected return on plan assets	(23,575)	(22,925)
Amortization of prior service cost	(1,525)	(1,650)
Amortization of net actuarial loss	11,175	3,350
Net periodic benefit cost	9,850	1,125
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	200	150
Interest cost on projected benefit obligation	650	675
Amortization of prior service cost	(350)	(350)
Amortization of net actuarial loss	25
Net periodic benefit cost	$ 525	$ 475