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Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Federal Clean Water Act Section 316(b)    
Manufactured Gas Plant (MGP) Site [Abstract]    
Minimum Number of Plants Which Could Be Required to Make Improvements to Reduce Entrainment 2  
Capital Addition Purchase Commitments [Member] | Federal Clean Water Act Section 316(b)    
Manufactured Gas Plant (MGP) Site [Abstract]    
Liability for Estimated Cost to Comply with Entrainment Regulation $ 4.0  
Ashland MGP Site    
Manufactured Gas Plant (MGP) Site [Abstract]    
Current Cost Estimate for Site Remediation 192.0  
Estimated Amount Spent on Cleanup 165.0  
Accrual for Environmental Loss Contingencies, Gross $ 27.0 $ 30.0
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, Landfill, or Disposal Sites [Domain]    
Manufactured Gas Plant (MGP) Site [Abstract]    
Number of Identified MGP sites Under Current Investigation and/or Remediation 2  
Liability for Estimated Cost of Remediating Site $ 1.7 $ 0.1