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, 2014	Mar. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,100	$ 6,050
Interest cost on projected benefit obligation	17,250	15,126
Expected return on plan assets	(22,925)	(21,875)
Amortization of prior service cost (credit)	(1,650)	(1,650)
Amortization of net actuarial loss	3,350	10,400
Net periodic benefit cost	1,125	8,051
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	150	200
Interest cost on projected benefit obligation	675	675
Expected return on plan assets
Amortization of prior service cost (credit)	(350)	(350)
Amortization of net actuarial loss		100
Net periodic benefit cost	$ 475	$ 625