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Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) (USD $)
Jun. 30, 2013
Dec. 31, 2012
Low [Member]
    
Commodity Contracts    
Natural gas financial swaps $ 3.38 $ 3.67
Electricity financial swaps 10.83 7.12
Electricity physical forward purchase 9.88  
High [Member]
    
Commodity Contracts    
Natural gas financial swaps 5.27 5.21
Electricity financial swaps 45.71 51.72
Electricity physical forward purchase 49.37  
Weighted Average [Member]
    
Commodity Contracts    
Natural gas financial swaps 4.12 4.28
Electricity financial swaps 37.31 41.14
Electricity physical forward purchase 35.00  
Assets [Member]
    
Commodity Contracts    
Natural gas financial swaps 0 2,000,000
Electricity financial swaps 0 0
Electricity physical forward purchase 0  
Total commodity contracts 0 2,000,000
Liabilities [Member]
    
Commodity Contracts    
Natural gas financial swaps 14,000,000 8,000,000
Electricity financial swaps 14,000,000 10,000,000
Electricity physical forward purchase 28,000,000  
Total commodity contracts $ 56,000,000 $ 18,000,000