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, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	$ (377)	$ 586	$ 590	$ (1,518)
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income	252	(1,736)	6	(3,196)
Natural gas contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	(377)	586	590	(1,518)
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	$ 252	$ (1,736)	$ 6	$ (3,196)