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Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Site
Dec. 31, 2016
USD ($)
Ashland MGP Site    
Manufactured Gas Plant (MGP) Site [Abstract]    
Number of Properties Not Owned Included in Superfund Site | Site 2  
Current Cost Estimate for Site Remediation $ 168.0  
Estimated Amount Spent on Cleanup 138.0  
Accrual for Environmental Loss Contingencies, Gross $ 30.0 $ 64.0
Approved Amortization Period for Recovery of Remediation Costs in Natural Gas Rates 10 years  
Carrying Cost Percentage to Be Applied to Unamortized Regulatory Asset 3.00%  
Other MGP Sites    
Manufactured Gas Plant (MGP) Site [Abstract]    
Number of Identified MGP sites Under Current Investigation and/or Remediation | Site 1  
Liability for Estimated Cost of Remediating Site $ 0.1 $ 0.0
PSCW Proceeding - Electric and Gas Rate Case 2016 - Gas Rates 2016 | Ashland MGP Site    
Manufactured Gas Plant (MGP) Site [Abstract]    
Public Utilities, Approved Annual Recovery, Collected Through Base Rates 12.0  
PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2017 | Ashland MGP Site    
Manufactured Gas Plant (MGP) Site [Abstract]    
Public Utilities, Approved Annual Recovery Collected Through Base Rates $ 18.0