EQUITY EQUITY (Details) - USD ($)	3 Months Ended													6 Months Ended	12 Months Ended
	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	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Offering Price of Common Stock	$ 54.50
General partner ownership interest														41.20%
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		20.00%													20.00%
Common Stock, Shares, Issued		423,166,234				245,811,180									423,166,234	245,811,180
Dividends, Common Stock															$ 829,400,000	$ 517,600,000	$ 509,200,000
Common Stock, Dividends, Per Share, Declared															$ 2.72	$ 2.46	$ 2.43
Common Stock, Dividends, Per Share, Cash Paid		$ 0.745	$ 0.745	$ 0.615	$ 0.615	$ 0.615	$ 0.615	$ 0.615	$ 0.615	$ 0.615	$ 0.605	$ 0.605	$ 0.605		$ 2.720	$ 2.460	$ 2.430
Stock Issued During Period, Shares, New Issues	21,900,000
Noncash Contribution Expense															$ 12,600,000	$ 0	$ 0
Dividends, Preferred Stock, Cash		$ 100,000													600,000
Preferred stock dividends paid															$ 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
Noncontrolling Interest, Increase from Subsidiary Equity Issuance																	$ 393,997,000
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. Under the Partnership Agreement, distributions were made to the partners with respect to each calendar quarter in an amount equal to 100 percent of available cash as defined in the Partnership Agreement. Available cash generally was distributed 98 percent to limited partners and 2 percent to the general partner. The general partner’s percentage interest in quarterly distributions were increased after certain specified target levels were met during the quarter. Under the incentive distribution provisions, as set forth in the Partnership Agreement, the general partner received:15 percent of amounts distributed in excess of $0.3025 per unit; 25 percent of amounts distributed in excess of $0.3575 per unit; and 50 percent of amounts distributed in excess of $0.4675 per unit.
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	$ 3.080
Common Stock, Dividends, Per Share, Declared	$ 0.770
Preferred stock dividends paid	$ 300,000
Dividends Payable, Date to be Paid	Jan. 29, 2018
Issued Under Equity Agreement
Common Stock Sold Under Equity Distribution Agreement															8,400,000
Issued Under Equity Agreement | Subsequent Event [Member]
Proceeds from Issuance or Sale of Equity															$ 448,300,000
Private Placement
Partners' Capital Account, Units, Sold in Private Placement											21,500,000
Per Unit Offering Price Under Private Placement											$ 30.17
Partners Capital Account Units Sold Under Equity Distribution Agreement											3,300,000
Proceeds from Sale of Interest in Partnership Unit											$ 749,000,000
Partners' Capital Account, Contributions											$ 15,300,000
Issued Under Equity Distribution Agreement
Partners Capital Account Units Sold Under Equity Distribution Agreement																	10,500,000
Proceeds from Sale of Interest in Partnership Unit																	$ 381,600,000
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 Declared
Distribution Made to Limited Partner, Distribution Date				May 01, 2017	Jan. 30, 2017
Distributions Per Unit				$ 0.79	$ 0.79
Distribution Paid
Distributions Per Unit															$ 1.58	$ 3.16	$ 3.16
Total distributions paid															$ 666,001,000	$ 1,332,003,000	$ 1,230,475,000
General Partner
General partner ownership interest														2.00%
ONEOK
Common Stock, Shares, Issued				168,900,000										168,900,000
Aggregate Amount Of Common Shares Available For Issuance And Sale Under Equity Distribution Agreement															1,000,000,000
General Partner | Distribution Paid
General partner distributions															13,320,000	26,640,000	24,610,000
Incentive distributions															201,076,000	402,152,000	371,500,000
Distributions to general partner															214,396,000	428,792,000	396,110,000
ONEOK | Distribution Paid
Limited partner distributions to ONEOK															180,646,000	361,292,000	310,230,000
Limited Partner | Distribution Paid
Limited partner distributions to other unitholders															$ 270,959,000	$ 541,919,000	$ 524,135,000