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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Jun. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Entity Information [Line Items]            
Common Stock, Shares, Issued 445,016,234     445,016,234   423,166,234
Business Acquisition, Description of Acquired Entity     On June 30, 2017, we completed the acquisition of all of the outstanding common units of ONEOK Partners that we did not already own at a fixed exchange ratio of 0.985 of a share of our common stock for each ONEOK Partners common unit. We issued 168.9 million shares of our common stock to third-party common unitholders of ONEOK Partners in exchange for all of the 171.5 million outstanding common units of ONEOK Partners that we previously did not own. As a result of the completion of the Merger Transaction, common units of ONEOK Partners are no longer publicly traded.      
General partnership ownership interest     41.20% 100.00%   100.00%
Impairment of long-lived assets $ 0 $ 15,970   $ 0 $ 15,970  
Equity Method Investment, Other than Temporary Impairment $ 0 $ 4,270   $ 0 $ 4,270