Stockholder's Equity - Schedule of Accumulated Other Comprehensive Income Comprises Changes in Equity (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	$ (182,093)	$ (97,307)	$ (132,530)
Other comprehensive income (loss)	5,051	(84,786)	35,223
Balance at end of year	(177,042)	(182,093)	(97,307)
Defined benefit pension plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	(192,552)	(116,745)	(158,401)
Net actuarial gain (loss) arising during year	922	(80,963)	31,388
Amortization of prior service cost	744	744	761
Amortization of net actuarial losses	11,155	4,412	9,507
Balance at end of year	(179,731)	(192,552)	(116,745)
OPEB [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	10,459	19,438	25,871
Net actuarial gain (loss) arising during year	(2,777)	(4,025)	(1,581)
Amortization of prior service cost	(9,766)	(9,765)	(9,990)
Amortization of net actuarial losses	4,773	4,811	5,138
Balance at end of year	$ 2,689	$ 10,459	$ 19,438