Pensions and Other Postretirement Benefits - Summary of Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined benefit pension plans [Member]
Changes In Plan Assets And Benefit Obligations Recognized In Other Comprehensive Income Loss [Line Items]
Net actuarial gain (loss) arising during the year	$ 1,270	$ (134,066)	$ 50,808
Amortization of prior service cost (credit)	1,232	1,232	1,232
Amortization of net actuarial losses	18,472	7,306	15,389
Total	20,974	(125,528)	67,429
OPEB [Member]
Changes In Plan Assets And Benefit Obligations Recognized In Other Comprehensive Income Loss [Line Items]
Net actuarial gain (loss) arising during the year	(4,425)	(6,664)	(2,561)
Amortization of prior service cost (credit)	(16,170)	(16,170)	(16,170)
Amortization of net actuarial losses	7,905	7,965	8,317
Total	$ (12,690)	$ (14,869)	$ (10,413)