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