Derivatives (Narrative - Commodity Future Contracts) (Details) (Cash Flow Hedging [Member], Natural gas contracts [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012 MMBTU	Jun. 30, 2011	Jun. 30, 2012 MMBTU	Jun. 30, 2011	Dec. 31, 2011 MMBTU
Derivative [Line Items]
Natural gas contracts, notional amounts (in millions of BTUs)	2,190,000		2,190,000		3,070,000
Derivative, additional cash paid on settlement of hedge	$ 1.7	$ 1.2	$ 3.2	$ 2.0
Cash flow hedge loss to be reclassified within 12 months			$ 2.0
Minimum [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, percent of anticipated requirements	40.00%		40.00%
Maximum [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, percent of anticipated requirements	70.00%		70.00%
Forecast of commodity requirements, maximum length of time used			18 months