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EQUITY EQUITY (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Jun. 30, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Public Offering Price of Common Stock |  | $ 54.50 |  |  |  |  |  |  |  |  |  |  |  |  | $ 54.50 |  |
General partner ownership interest |  |  |  |  |  |  |  |  |  |  |  |  |  | 41.20% | 100.00% |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  |  | 20.00% |
Business Combination, Consideration Transferred |  |  | $ 195,000,000 |  |  |  |  |  |  |  |  |  |  |  | $ 195,000,000 |  |
Common Stock, Shares, Issued |  | 445,016,234 |  |  |  | 423,166,234 |  |  |  |  |  |  |  |  | 445,016,234 | 423,166,234 |
Dividends, Cash |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,335,100,000 | $ 829,400,000 | $ 517,600,000
Common Stock, Dividends, Per Share, Cash Paid |  | $ 0.855 | $ 0.825 | $ 0.795 | $ 0.770 | $ 0.745 | $ 0.745 | $ 0.615 | $ 0.615 | $ 0.615 | $ 0.615 | $ 0.615 | $ 0.615 |  | $ 3.245 | $ 2.720 | $ 2.460
Stock Issued During Period, Shares, New Issues |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21,900,000 |  |
Noncash Contribution Expense |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 | $ 12,600,000 | $ 0
Dividends, Preferred Stock, Cash |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,100,000 | 600,000 |
Preferred stock dividends paid |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,100,000 | $ 767,000 |
Proceeds from Issuance or Sale of Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,200,000,000 |  |
Aggregate Amount Of Common Units Available For Issuance And Sale Under Equity Distribution Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 650,000,000 |  |  | $ 650,000,000
Key Provisions of Operating or Partnership Agreement, Description |  |  |  |  |  |  |  |  |  |  |  |  |  | Cash Distributions - Prior to the consummation of the Merger Transaction, we received distributions from ONEOK Partners on our common and Class B units and our 2 percent general partner interest, which included our incentive distribution rights. |  |  |
Issued Under Equity Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Sold Under Equity Distribution Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |
Issued Under Equity Distribution Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Partners Capital Account Units Sold Under Equity Distribution Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  | 0
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Declared, Annualized | $ 3.440 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Declared | $ 0.860 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock dividends paid | $ 300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable, Date to be Paid | Jan. 28, 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issued Under Equity Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Sold Under Equity Distribution Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,400,000 |
Proceeds from Issuance or Sale of Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 448,300,000 |
Series A Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Outstanding |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |
Series B Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Outstanding |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |
Series E Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20,000 |
Stock Issued During Period, Value, New Issues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,000,000 |
Noncash Contribution Expense |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20,000,000 |
Preferred Stock, Dividend Rate, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.50% |  |
Distribution Paid |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Distributions Per Unit |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.58 |  |  | $ 3.16
Total distributions paid |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 666,001,000 |  |  | $ 1,332,003,000
ONEOK |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate Amount Of Common Shares Available For Issuance And Sale Under Equity Distribution Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000,000,000 |
General Partner |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General partner ownership interest |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |  |  |
General Partner | Distribution Paid |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General partner distributions |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 13,320,000 |  |  | 26,640,000
Incentive distributions |  |  |  |  |  |  |  |  |  |  |  |  |  | 201,076,000 |  |  | 402,152,000
Distributions to general partner |  |  |  |  |  |  |  |  |  |  |  |  |  | 214,396,000 |  |  | 428,792,000
ONEOK | Distribution Paid |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Limited partner distributions to ONEOK |  |  |  |  |  |  |  |  |  |  |  |  |  | 180,646,000 |  |  | 361,292,000
Limited Partner | Distribution Paid |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Limited partner distributions to other unitholders |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 270,959,000 |  |  | $ 541,919,000

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