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Derivatives (Narrative - Commodity Future Contracts) (Details) (Cash Flow Hedging [Member], Natural gas contracts [Member], USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2014
MMBTU
Sep. 30, 2014
MMBTU
Sep. 30, 2013
Dec. 31, 2013
MMBTU
Derivative [Line Items]        
Natural gas contracts, notional amounts (in millions of BTUs) 3,650,000 3,650,000   1,520,000
Loss on Cash Flow Hedge Ineffectiveness     $ (0.3)  
Derivative, additional cash paid (received) on settlement of hedge 0.1 (0.8)    
Cash flow hedge loss to be reclassified within 12 months   $ 0.4    
Minimum [Member]
       
Derivative [Line Items]        
Forecast of anticipated requirements, percentage of forecast eligible for hedging 40.00% 40.00%    
Maximum [Member]
       
Derivative [Line Items]        
Forecast of anticipated requirements, percentage of forecast eligible for hedging 70.00% 70.00%    
Forecast of commodity requirements, maximum length of time used   18 months