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Derivatives (Narrative - Commodity Future Contracts) (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2016
USD ($)
MMBTU
Mar. 31, 2015
USD ($)
Dec. 31, 2015
MMBTU
Derivative [Line Items]      
Contracts where hedge accounting was not elected $ (418) $ 74  
Cash Flow Hedging [Member] | Natural Gas Contracts      
Derivative [Line Items]      
Forecast of commodity requirements, maximum length of time used 18 months    
Natural gas contracts, notional amounts (in millions of BTUs) | MMBTU 2,220,000   3,000,000
Derivative, Additional Cash Paid on Settlement of Hedge $ 1,200 1,200  
Cash flow hedge loss to be reclassified within 12 months $ 1,100    
Minimum [Member] | Cash Flow Hedging [Member] | Natural Gas Contracts      
Derivative [Line Items]      
Forecast of anticipated requirements, percentage of forecast eligible for hedging 40.00%    
Maximum [Member] | Cash Flow Hedging [Member] | Natural Gas Contracts      
Derivative [Line Items]      
Forecast of anticipated requirements, percentage of forecast eligible for hedging 70.00%    
Other Income (Expense) [Member] | Cash Flow Hedging [Member] | Natural Gas Contracts      
Derivative [Line Items]      
Contracts where hedge accounting was not elected $ 370 $ (295)