Derivatives (Effective Portion of Derivative Gain Loss Reclassified from Accumulated OCI into Income) (Details) (Designated as Hedging Instrument, Cash Flow Hedging, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income	$ (4,707)	$ (4,639)	$ (8,962)
Natural Gas Contracts | Cost of Sales
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income	$ (4,707)	$ (4,639)	$ (8,962)