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COMMITMENTS AND CONTINGENCIES (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
USD ($)
Dec. 31, 2016
Commitments and Contingencies [Line Items]      
Number Of Former Manufactured Gas Sites Where We Own Or Retain Legal Responsibility For Environmental Conditions 12    
AAO associated with MGP costs requested amount, cap   $ 15.0  
Regulatory asset expected to be recorded for MGP costs accrued at January 1, 2017   $ 5.9  
Number of sites where we have completed or addressed removal of the source of soil contamination according to plans approved by KDHE. 11    
Number of sites with ongoing groundwater monitoring 8    
Number of sites where regulatory closure has been achieved 3    
Environmental Reserve Estimate Range, Low 5.6   4
Environmental Reserve Estimate Range, High 7   7
Environmental Reserve Estimate, Actual 1.6    
Percentage yield of high consequence pipeline areas 30.00%