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Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2016
USD ($)
Site
Dec. 31, 2017
USD ($)
Dec. 31, 2015
USD ($)
Ashland Manufactured Gas Plant (MGP) Site [Abstract]      
Liability for estimated cost of remediating sites, current $ 17,453   $ 17,155
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 $ 94,200   94,400
Liability for estimated cost of remediating sites, current 17,200   17,000
Public Utilities, Annual recovery collected through base rates     $ 4,700
Ashland MGP Site - Phase I Project Area      
Ashland Manufactured Gas Plant (MGP) Site [Abstract]      
Accrual for Environmental Loss Contingencies, Gross 68,000    
Estimated amount spent on cleanup $ 51,000    
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%    
Ashland MGP Site - Sediments      
Ashland Manufactured Gas Plant (MGP) Site [Abstract]      
Estimated cost of remediating site, low end of range $ 63,000    
Estimated cost of remediating site, high end of range $ 77,000    
Potential percent of increase to the high end of the range of estimated site remediation costs (as a percent) 50.00%    
Potential percent of decrease to the low end of the range of estimated site remediation costs (as a percent) 30.00%    
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 $ 7,600    
Other PRPs | Ashland MGP Site      
Ashland Manufactured Gas Plant (MGP) Site [Abstract]      
Number of PRPs that have contributed to remediation site     3
Contributions to site cleanup by PRPs     $ 15,900
Scenario, Forecast [Member] | PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2017 [Member] | Ashland MGP Site      
Ashland Manufactured Gas Plant (MGP) Site [Abstract]      
Public Utilities, Requested annual recovery collected through base rates   $ 12,400