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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	$ (124)	$ (377)	$ 506	$ 590
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income	349	252	814	6
Natural gas contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	(124)	(377)	506	590
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	$ 349	$ 252	$ 814	$ 6