Effect of Derivative Instruments on the Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion)		$ (269)		$ (905)
Amount Reclassified from Accumulated OCI into Income		1,673		13,013
Interest rate swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion)		(20)		(447)
Natural gas swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion)		(249)		(458)
Interest expense, net of amounts capitalized [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Reclassified from Accumulated OCI into Income		1,549		12,674
Operating Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Reclassified from Accumulated OCI into Income		$ 124		$ 339