Derivative Financial Instruments (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Effect of derivative instruments on the statement of operations
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	$ (980)	$ (7,241)
Amount Reclassified from Accumulated OCI into Income	13,433	20,449
Operating Costs [Member]
Effect of derivative instruments on the statement of operations
Amount Reclassified from Accumulated OCI into Income	759	295
Interest expense, net of amounts capitalized [Member]
Effect of derivative instruments on the statement of operations
Amount Reclassified from Accumulated OCI into Income	12,674	20,154
Interest rate swaps [Member]
Effect of derivative instruments on the statement of operations
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(447)	(6,720)
Natural gas swaps [Member]
Effect of derivative instruments on the statement of operations
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(533)	(521)
Natural gas swaps [Member] | Other gains (losses), net [Member]
Effect of derivative instruments on the statement of operations
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ 0	$ 202