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Derivatives (Narrative - Commodity Future Contracts) (Details) (Cash Flow Hedging [Member], Natural gas contracts [Member], USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2013
MMBTU
Jun. 30, 2012
Jun. 30, 2013
MMBTU
Jun. 30, 2012
Dec. 31, 2012
MMBTU
Derivative [Line Items]          
Natural gas contracts, notional amounts (in millions of BTUs) 1,550,000   1,550,000   2,400,000
Loss on cash flow hedge ineffectiveness $ 300,000   $ 300,000    
Derivative, additional cash paid (received) on settlement of hedge (300,000) 1,700,000 0 3,200,000  
Cash flow hedge loss to be reclassified within 12 months     $ 100,000    
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