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Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details)
$ in Millions
1 Months Ended 12 Months Ended
Feb. 28, 2017
Dec. 31, 2016
USD ($)
Site
Dec. 31, 2016
USD ($)
Site
Dec. 31, 2015
USD ($)
Ashland MGP Site        
Manufactured Gas Plant (MGP) Site [Abstract]        
Number of Properties Not Owned Included in Superfund Site | Site   2 2  
Accrual for Environmental Loss Contingencies, Gross   $ 64.3 $ 64.3 $ 94.4
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%  
Ashland MGP Site - Phase I Project Area        
Manufactured Gas Plant (MGP) Site [Abstract]        
Accrual for Environmental Loss Contingencies, Gross   72.4 $ 72.4  
Estimated Amount Spent on Cleanup   $ 56.7 $ 56.7  
Other MGP Sites        
Manufactured Gas Plant (MGP) Site [Abstract]        
Number of Identified MGP sites Under Current Investigation and/or Remediation | Site   1 1  
Liability for Estimated Cost of Remediating Site   $ 0.1 $ 0.1 $ 0.2
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, For 2016, Collected Through Base Rates     $ 7.6  
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   $ 12.4    
Subsequent Event | Ashland MGP Site Phase II Project Area        
Manufactured Gas Plant (MGP) Site [Abstract]        
Length of Public Comment Period that Lapsed 30 days