SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Entity [Line Items]
Equity Method Investment, Ownership Percentage		50.00%
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.
Deferred Tax Assets, Gross, Noncurrent	$ 2,100,000
Deferred Tax Asset, Tax Basis	8,800,000
Deferred Tax Asset, Book Basis	$ 3,000,000
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	35.00%	35.00%
General partner ownership interest	41.20%	100.00%
Common stock		$ 1,203,981	$ 471,358	$ 21,971
Accumulated other comprehensive loss			(40,288)
Noncontrolling interests in consolidated subsidiaries			$ 2,146,462
Number Of Years Of Service Employees Must Work To Be Entitled To Postretirement Medical And Life Insurance Benefits			5 years
ONEOK
Entity [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	37.00%
Common stock	$ 1,700
Paid-in capital	5,228,600
Accumulated other comprehensive loss	(40,300)
Noncontrolling interests in consolidated subsidiaries	(3,043,500)
Deferred income taxes	$ (2,146,500)