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Derivatives (Narrative - Commodity Future Contracts) (Details) - Cash Flow Hedging [Member] - Natural Gas Contracts
$ in Millions
3 Months Ended
Mar. 31, 2017
USD ($)
MMBTU
Mar. 31, 2016
USD ($)
Dec. 31, 2016
MMBTU
Derivative [Line Items]      
Forecast of commodity requirements, maximum length of time used 18 months    
Natural gas contracts, notional amounts (in millions of BTUs) | MMBTU 2,170,000   2,590,000
Derivative, Cash Received on Hedge $ (0.1)    
Derivative, Additional Cash Paid on Settlement of Hedge   $ 1.2  
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months $ 0.2    
Minimum [Member]      
Derivative [Line Items]      
Forecast of anticipated requirements, percentage of forecast eligible for hedging 40.00%    
Maximum [Member]      
Derivative [Line Items]      
Forecast of anticipated requirements, percentage of forecast eligible for hedging 70.00%