Employee Benefit Plans - Schedule of Components of Net Periodic Costs or Credits (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Defined Benefit Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,292	$ 1,168
Interest cost	4,022	4,385
Expected return on plan assets	(13,451)	(12,202)
Amortization of accumulated other comprehensive income - Prior service cost	308	308
Amortization of accumulated other comprehensive income - Actuarial losses	4,133	4,442
Total credit	(3,696)	(1,899)
OPEB [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	46	40
Interest cost	459	507
Amortization of accumulated other comprehensive income - Prior service cost	(4,043)	(4,043)
Amortization of accumulated other comprehensive income - Actuarial losses	1,864	1,919
Total credit	$ (1,674)	$ (1,577)