Derivatives (Narrative - Commodity Future Contracts) (Details) (Cash Flow Hedging, Natural Gas Contracts, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 MMBTU	Dec. 31, 2012 MMBTU	Dec. 31, 2011
Derivative [Line Items]
Natural gas contracts, notional amounts (in millions of BTUs)	1,520,000	2,400,000
Loss on cash flow hedge ineffectiveness	$ 0.3
Derivative, additional cash paid on settlement of hedge	0.1	4.7	4.7
Cash flow hedge gain to be reclassified within 12 months	$ 0.4
Minimum [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, percent of anticipated requirements	40.00%
Maximum [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, percent of anticipated requirements	70.00%
Forecast of commodity requirements, maximum length of time used	18 months