Derivatives (Effective Portion of Derivative Gain Loss) (Details) (Designated as Hedging Instrument [Member], Cash Flow Hedging [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	$ 967	$ (2,104)
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income	(246)	(1,460)
Natural gas contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	967	(2,104)
Natural gas contracts [Member] | Cost of Sales [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income	$ (246)	$ (1,460)