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Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2016
USD ($)
Site
Dec. 31, 2017
USD ($)
Dec. 31, 2015
USD ($)
Ashland MGP Site      
Ashland Manufactured Gas Plant (MGP) Site [Abstract]      
Number of properties not owned included in superfund site | Site 2    
Accrual for Environmental Loss Contingencies, Gross $ 84.6   $ 94.4
Ashland MGP Site - Phase I Project Area      
Ashland Manufactured Gas Plant (MGP) Site [Abstract]      
Accrual for Environmental Loss Contingencies, Gross 71.4    
Estimated amount spent on cleanup $ 52.6    
Approved amortization period for recovery of remediation costs in natural gas rates (in years) 10 years    
Carrying cost percentage to be applied to unamortized regulatory asset 3.00%    
PSCW Proceeding - Electric and Gas Rate Case 2016 - Gas Rates 2016 | Ashland MGP Site      
Ashland Manufactured Gas Plant (MGP) Site [Abstract]      
Public Utilities, Approved annual recovery collected through base rates $ 7.6    
Scenario, Forecast [Member] | PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2017 | Ashland MGP Site      
Ashland Manufactured Gas Plant (MGP) Site [Abstract]      
Public Utilities, Requested annual recovery collected through base rates   $ 12.4