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Derivatives (Narrative - Commodity Future Contracts) (Details)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2016
USD ($)
MMBTU
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
MMBTU
Sep. 30, 2015
USD ($)
Dec. 31, 2015
MMBTU
Derivative [Line Items]          
Gain (loss) on contracts where hedge accounting was not elected $ 106 $ 135 $ (281) $ (152)  
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 2,510,000   2,510,000   3,000,000
Derivative, Additional Cash Paid on Settlement of Hedge $ 100 1,000 $ 2,300 3,200  
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months     $ 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%   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%   70.00%    
Other Income (Expense) [Member] | Cash Flow Hedging [Member] | Natural Gas Contracts          
Derivative [Line Items]          
Gain (loss) on contracts where hedge accounting was not elected $ 11 $ (222) $ 1,150 $ (459)