Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,090	$ 7,387	$ 18,270	$ 22,162
Interest cost on projected benefit obligation	15,032	15,509	45,097	46,527
Expected return on plan assets	(21,838)	(17,627)	(65,515)	(52,883)
Amortization of prior service cost	(1,639)	(1,640)	(4,917)	(4,919)
Amortization of net actuarial loss	10,269	9,291	30,807	27,874
Net periodic benefit cost	7,914	12,920	23,742	38,761
Other postretirement benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	186	167	557	501
Interest cost on projected benefit obligation	673	934	2,018	2,803
Expected return on plan assets
Amortization of prior service cost	(340)	5	(1,019)	15
Amortization of net actuarial loss	90	133	270	398
Net periodic benefit cost	$ 609	$ 1,239	$ 1,826	$ 3,717