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Nuclear Plant (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Investments in decommissioning trust fund [Line Items]            
Cost Basis $ 166.6   $ 166.6   $ 160.5  
Unrealized Gains 81.9   81.9   64.4  
Unrealized Losses (1.0)   (1.0)   (2.0)  
Fair Value 247.5   $ 247.5   222.9 $ 200.7
Weighted average maturity of debt securities (in years)     9 years      
Realized gains 0.7 $ 0.6 $ 1.1 $ 1.5    
Realized losses (0.5) $ (0.3) (0.6) $ (1.3)    
Equity Securities [Member]            
Investments in decommissioning trust fund [Line Items]            
Cost Basis 95.8   95.8   93.3  
Unrealized Gains 79.2   79.2   62.1  
Unrealized Losses (0.8)   (0.8)   (1.5)  
Fair Value 174.2   174.2   153.9  
Debt Securities [Member]            
Investments in decommissioning trust fund [Line Items]            
Cost Basis 67.3   67.3   63.4  
Unrealized Gains 2.7   2.7   2.3  
Unrealized Losses (0.2)   (0.2)   (0.5)  
Fair Value 69.8   69.8   65.2  
Other Securities [Member]            
Investments in decommissioning trust fund [Line Items]            
Cost Basis 3.5   3.5   3.8  
Fair Value $ 3.5   $ 3.5   $ 3.8