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EQUITY (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2017
Jun. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners                 20.00%
Business Combination, Consideration Transferred   $ 195,000              
Stock Issued During Period, Shares, New Issues       21,900,000          
Share Price       $ 54.50          
Proceeds from Sale of Common Stock       $ 1,200,000          
Common Stock, Dividends, Per Share, Cash Paid   $ 0.825 $ 0.795 $ 0.77     $ 2.390 $ 1.975  
Common Stock, Dividends, Per Share, Declared   $ 0.825     $ 0.745   $ 2.390 $ 1.975  
Dividends, Preferred Stock, Cash   $ 300 $ 300 $ 300          
Preferred Stock, Dividends, Declared             $ 825 $ 493  
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.      
Partnership Capital Distributions [Line Items]                  
General partnership ownership interest           41.20% 100.00%   100.00%
Issued Under Equity Agreement [Member]                  
Proceeds from Sale of Common Stock                 $ 448,300
Common Stock Sold Under Equity Distribution Agreement             0   8,400,000
Series E Preferred Stock [Member]                  
Preferred Stock, Dividend Rate, Percentage             5.50%    
Subsequent Event [Member]                  
Common Stock, Dividends, Per Share, Declared $ 0.855                
Dividends Payable, Date of Record Nov. 05, 2018                
Dividends Payable, Date to be Paid Nov. 14, 2018                
Preferred Stock, Dividends, Declared $ 300                
General Partner [Member]                  
Partnership Capital Distributions [Line Items]                  
General partnership ownership interest           2.00%      
ONEOK [Member]                  
Aggregate Amount Of Common Shares Available For Issuance And Sale Under Equity Distribution Agreement             $ 1,000,000