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, 2014	Mar. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	$ 630	$ 967
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income	465	(246)
Natural gas contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	630	967
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	$ 465	$ (246)