Derivatives (Narrative - Commodity Future Contracts) (Details) (Cash Flow Hedging, Natural Gas Contracts, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 MMBTU	Dec. 31, 2011 MMBTU	Dec. 31, 2010
Derivative [Line Items]
Natural gas contracts, notional amounts (in millions of BTUs)	2,400,000	3,070,000
Derivative, additional cash paid on settlement of hedge	$ 4.7	$ 4.7	$ 8.6
Cash flow hedge loss to be reclassified within 12 months	$ 0.4
Minimum
Derivative [Line Items]
Derivative, nonmonetary notional amount, percent of anticipated requirements	40.00%
Maximum
Derivative [Line Items]
Derivative, nonmonetary notional amount, percent of anticipated requirements	70.00%
Forecast of commodity requirements, maximum length of time used	18 months