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Derivatives (Narrative - Commodity Future Contracts) (Details) - Cash Flow Hedging - Natural Gas Contracts
3 Months Ended
Mar. 31, 2018
Derivative [Line Items]  
Forecast of commodity requirements, maximum length of time used 18 months
Minimum  
Derivative [Line Items]  
Forecast of anticipated requirements, percentage of forecast eligible for hedging 40.00%
Maximum  
Derivative [Line Items]  
Forecast of anticipated requirements, percentage of forecast eligible for hedging 70.00%