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Contingencies (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 1997
Dec. 31, 2016
Sep. 30, 2008
Dec. 31, 1993
Loss Contingencies [Line Items]            
Malpractice Loss Contingency, Letters of Credit and Surety Bonds $ 145.6          
Long-term Purchase Commitment, Amount   $ 200.0        
Loss Contingency, Damages Sought, Value   1,200.0        
Liability for Title Claims and Claims Adjustment Expense   $ 8.0        
Site Contingency, Names of Other Potentially Responsible Parties   100 69      
Litigation Settlement, Expense   $ 115.0        
Loss Contingency, Range of Possible Loss, Maximum   1,700.0        
Loss Contingency, Range of Possible Loss, Portion Not Accrued   500.0        
Loss Contingency, Inestimable Loss   1,100.0        
investment in Trojan           87.00%
Class action damages sought   260.0        
Refund to customers for Trojan Investment including interest         $ 33.0  
Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base 514.0          
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment 637.0          
Loss Contingency, Accrual, Current   $ 14.0   $ 3.0    
Minimum [Member]            
Loss Contingencies [Line Items]            
Loss Contingency, Damages Sought, Value 44.0          
Maximum [Member]            
Loss Contingencies [Line Items]            
Loss Contingency, Damages Sought, Value $ 117.0