Derivatives (Effective Portion of Derivative Gain Loss Recognized in Other Comprehensive 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) recognized in other comprehensive income (loss)	$ (1,439)	$ (5,263)	$ (6,362)
Natural Gas Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	$ (1,439)	$ (5,263)	$ (6,362)