Pension and Other Postretirement Benefits - Schedule of Components of Net Periodic Costs or Credits (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined benefit pension plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 4,593	$ 3,419	$ 3,218
Interest cost	17,664	19,151	20,161
Expected return on plan assets	(48,819)	(55,496)	(44,654)
Amortization of prior service cost (credit)	1,232	1,232	1,232
Amortization of net actuarial losses	18,472	7,306	15,389
Total benefit credit	(6,858)	(24,388)	(4,654)
OPEB [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	154	118	106
Interest cost	2,036	2,288	2,489
Amortization of prior service cost (credit)	(16,170)	(16,170)	(16,170)
Amortization of net actuarial losses	7,905	7,965	8,317
Total benefit credit	$ (6,075)	$ (5,799)	$ (5,258)