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Acquisitions - Approval Conditions for Integrys Acquisition (Details) - Integrys - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Jun. 30, 2015
Dec. 31, 2016
Jun. 29, 2015
Earnings sharing mechanisms      
Business Acquisition [Line Items]      
Expense for earnings sharing mechanisms   $ 24.4  
WE      
Business Acquisition [Line Items]      
Duration of earnings cap condition imposed by the PSCW (in years) 3 years    
Percentage of first 50 basis points to be shared with customers     50.00%
ROE in excess of authorized amount (as a percent)     0.50%
WG      
Business Acquisition [Line Items]      
Duration of earnings cap condition imposed by the PSCW (in years) 3 years    
Percentage of first 50 basis points to be shared with customers     50.00%
ROE in excess of authorized amount (as a percent)     0.50%
PGL      
Business Acquisition [Line Items]      
Duration of base rate freeze condition imposed by the ICC (in years) 2 years    
NSG      
Business Acquisition [Line Items]      
Duration of base rate freeze condition imposed by the ICC (in years) 2 years