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Fair Value of FInancial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Minimum [Member]    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Electricity physical forward purchase $ 7.79 $ 14.25
Natural gas financial swaps 1.26 1.85
Fnancial swaps - electricity 7.79 8.57
Maximum [Member]    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Electricity physical forward purchase 41.23 54.73
Natural gas financial swaps 2.92 4.92
Fnancial swaps - electricity 29.74 33.60
Weighted Average [Member]    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Electricity physical forward purchase 30.95 38.18
Natural gas financial swaps 1.90 2.64
Fnancial swaps - electricity 21.74 25.10
Assets [Member]    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Electricity physical forward purchase 0 0
Natural gas financial swaps 0 1,000,000
Fnancial swaps - electricity 0 1,000,000
Total commodity contracts 0 2,000,000
Liabilities [Member]    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Electricity physical forward purchase 130,000,000 112,000,000
Natural gas financial swaps 9,000,000 9,000,000
Fnancial swaps - electricity 0 0
Total commodity contracts $ 139,000,000 $ 121,000,000