DEBT (Details) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2015 USD ($) Rate	Sep. 30, 2017 USD ($) Rate	Sep. 30, 2015 USD ($) Rate	Dec. 31, 2017 USD ($) Rate	Dec. 31, 2016 USD ($) Rate	Dec. 31, 2015 USD ($)	Mar. 31, 2018 USD ($)
Debt Instrument [Line Items]
Impairment of long-lived assets				$ 15,970	$ 0	$ 83,673
Long-term Debt, Gross				9,198,677	9,489,057
Unamortized Portion of Terminated Swaps				18,468	20,186
Unamortized debt issuance costs and discounts				(78,193)	(68,320)
Current maturities of long-term debt				(432,650)	(410,650)
Short-term borrowings				(614,673)	(1,110,277)
Long-term debt				8,091,629	7,919,996
Senior Notes, Noncurrent		$ 1,200,000		1,200,000
Payments to Acquire Interest in Subsidiaries and Affiliates			$ 650,000
Long-term Debt, by Maturity [Abstract]
2018				432,700
2019				1,007,700
2020				307,700
2021				7,700
2022				1,453,200
ONEOK, Inc. Commercial Paper Program [Domain]
Debt Instrument [Line Items]
Commercial Paper				614,673	0
Maximum Amount Of Commercial Paper				2,500,000
ONEOK Partners Commercial Paper Program [Domain]
Debt Instrument [Line Items]
Maximum Amount Of Commercial Paper		2,400,000
$2.5 Billion Credit Agreement [Member]
Debt Instrument [Line Items]
Letters of Credit Outstanding, Amount				15,800
Line of credit facility sublimit				100,000
Line of Credit Facility Swingline Subfacility				200,000
Line Of Credit Facility Option To Increase Borrowing Capacity				$ 3,500,000
Line of Credit Facility, Interest Rate Description				borrowings, if any, will accrue at LIBOR plus 110 basis points
Line Of Credit Facility, Annual Facility Fee Description				the annual facility fee is 15 basis points
Indebtedness To Adjusted Ebitda Current				4.5
Debt instrument covenant description				Among other things, these covenants include maintaining a ratio of indebtedness to adjusted EBITDA (EBITDA, as defined in our $2.5 Billion Credit Agreement, adjusted for all noncash charges and increased for projected EBITDA from certain lender-approved capital expansion projects) of no more than 5.75 to 1 at December 31, 2017, and for the following quarter; 5.5 to 1 for the subsequent two quarters; and 5.0 to 1 thereafter. Once the covenant decreases to 5.0 to 1, if we consummate one or more acquisitions in which the aggregate purchase is $25 million or more, the allowable ratio of indebtedness to adjusted EBITDA will increase to 5.5 to 1 for the quarter in which the acquisition is completed and the two following quarters. The $2.5 Billion Credit Agreement includes a $100 million sublimit for the issuance of standby letters of credit and a $200 million sublimit for swingline loans. Under the terms of the $2.5 Billion Credit Agreement, we may request an increase in the size of the facility to an aggregate of $3.5 billion by either commitments from new lenders or increased commitments from existing lenders. The $2.5 Billion Credit Agreement contains provisions for an applicable margin rate and an annual facility fee, both of which adjust with changes in our credit ratings. Based on our current credit ratings, borrowings, if any, will accrue at LIBOR plus 110 basis points, and the annual facility fee is 15 basis points. We have the option to request two one-year extensions, subject to lender approval, which may be used for working capital, capital expenditures, acquisitions and mergers, the issuance of letters of credit and for other general corporate purposes. At December 31, 2017, our ratio of indebtedness to adjusted EBITDA was 4.5 to 1, and we were in compliance with all covenants under the $2.5 Billion Credit Agreement.
$2.5 Billion Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				$ 2,500,000
ONEOK Credit Agreement [Domain]
Debt Instrument [Line Items]
Letters of Credit Outstanding, Amount					1,100
Partnership Credit Agreement [Member]
Debt Instrument [Line Items]
Letters of Credit Outstanding, Amount					14,000
Senior Notes [Member]
Debt Instrument [Line Items]
Proceeds from Debt, Net of Issuance Costs		1,180,000
Note Payable from Public Offering Due 2018 [Member]
Long-term Debt, by Maturity [Abstract]
2018				425,000
Notes Payables due 2019 [Member]
Long-term Debt, by Maturity [Abstract]
2019				1,000,000
Note Payable from Public Offering Due 2020 [Member]
Long-term Debt, by Maturity [Abstract]
2020				300,000
Note Payable 2 from Public Offering Due 2022 [Member]
Long-term Debt, by Maturity [Abstract]
2022				1,447,400
Note Payable Due 2021 [Domain]
Long-term Debt, by Maturity [Abstract]
2021				$ 0
Partnership Interest [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Senior Notes, Noncurrent	$ 800,000
Proceeds from Debt, Net of Issuance Costs	792,300
Partnership Interest [Member] | Note Payable from Public Offering Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 300,000
Interest rates (in hundredths) | Rate	3.80%
Partnership Interest [Member] | Note Payable from Public Offering Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 500,000
Interest rates (in hundredths) | Rate	4.90%
Parent Company | Note Payable Due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			500,000
Proceeds from Debt, Net of Issuance Costs			$ 487,100
Interest rates (in hundredths) | Rate			7.50%
Guardian Pipeline [Member]
Debt Instrument [Line Items]
Debt instrument covenant description				Guardian Pipeline’s senior notes contain financial covenants that require the maintenance of certain financial ratios as defined in the master shelf agreement based on Guardian Pipeline’s financial position and results of operations. Upon any breach of these covenants, all amounts outstanding under the master shelf agreement may become due and payable immediately
Debt Instrument, Covenant Compliance				At December 31, 2017, Guardian Pipeline was in compliance with its financial covenants
Guardian Pipeline [Member] | Notes Payables due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 36,607	44,257
Average interest rate (in hundredths) | Rate				7.85%
Long-term Debt, by Maturity [Abstract]
2018				$ 7,700
2019				7,700
2020				7,700
2021				7,700
2022				$ 5,800
Parent Company
Debt Instrument [Line Items]
Short-term Debt, Weighted Average Interest Rate, at Point in Time | Rate				2.23%
Parent Company | Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument covenant description				The indentures governing ONEOK’s 6.875 percent senior notes due 2028 include an event of default upon acceleration of other indebtedness of $15 million or more, and the indentures governing the senior notes due 2022, 2023, 2027, 2035 and 2047 include an event of default upon the acceleration of other indebtedness of $100 million or more. Such events of default would entitle the trustee or the holders of 25 percent in aggregate principal amount of the outstanding senior notes due 2022, 2023, 2027, 2028, 2035 and 2047 to declare those senior notes immediately due and payable in full. The indenture for the notes due 2023 also contains a provision that allows the holders of the notes to require ONEOK to offer to repurchase all or any part of their notes if a change of control and a credit rating downgrade occur at a purchase price of 101 percent of the principal amount, plus accrued and unpaid interest, if any.
Debt instrument call feature				ONEOK may redeem its senior notes, in whole or in part, at any time prior to their maturity at a redemption price equal to the principal amount, plus accrued and unpaid interest and a make-whole premium. ONEOK may redeem the remaining balance of its senior notes due 2022, 2023, 2027 and 2047 at a redemption price equal to the principal amount, plus accrued and unpaid interest, starting three to six months before the maturity date as stipulated in the respective contract terms. ONEOK’s senior notes are senior unsecured obligations, ranking equally in right of payment with all of ONEOK’s existing and future unsecured senior indebtedness.
Parent Company | Note Payable Due 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 500,000
Interest rates (in hundredths) | Rate		4.00%
Parent Company | Notes Payables due 2047 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 700,000
Interest rates (in hundredths) | Rate		4.95%
Parent Company | Note Payable from Public Offering Due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 547,397	547,397
Interest rates (in hundredths) | Rate				4.25%
Parent Company | Note Payable Due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 500,000	500,000
Interest rates (in hundredths) | Rate				7.50%
Parent Company | Note Payable from Public Offering Due 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 500,000	0
Interest rates (in hundredths) | Rate				4.00%
Parent Company | Note Payables 1 due 2028 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 0	87,126
Repayment of Senior Notes		$ 87,000
Interest rates (in hundredths) | Rate		6.50%		6.50%
Parent Company | Note Payables 2 due 2028 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 100,000	100,000
Interest rates (in hundredths) | Rate				6.875%
Parent Company | Notes Payables due 2035 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 400,000	400,000
Interest rates (in hundredths) | Rate				6.00%
Parent Company | Note Payable from Public Offering Due 2047 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 700,000	0
Interest rates (in hundredths) | Rate				4.95%
Partnership Interest [Member]
Debt Instrument [Line Items]
Commercial Paper				$ 0	1,110,277
Partnership Interest [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument covenant description				ONEOK Partners’ senior notes are governed by an indenture containing covenants including, among other provisions, limitations on ONEOK Partners’ ability to place liens on its property or assets and to sell and lease back its property. The indenture includes an event of default upon acceleration of other indebtedness of $100 million or more. Such events of default would entitle the trustee or the holders of 25 percent in aggregate principal amount of any of ONEOK Partners’ outstanding senior notes to declare those notes immediately due and payable in full.
Debt instrument call feature				The senior notes may be redeemed, in whole or in part, at any time prior to their maturity at a redemption price equal to the principal amount, plus accrued and unpaid interest and a make-whole premium. The senior notes due 2018, 2020, 2022, 2023, 2025, 2041 and 2043 may be redeemed at a redemption price equal to the principal amount, plus accrued and unpaid interest, starting one to six months before their maturity dates as stipulated in the respective contract terms.
Partnership Interest [Member] | Note Payable from Public Offering Due 2017 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 0	400,000
Repayment of Senior Notes		$ 400,000
Interest rates (in hundredths) | Rate		2.00%		2.00%
Partnership Interest [Member] | Note Payable from Public Offering Due 2018 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 425,000	425,000
Interest rates (in hundredths) | Rate				3.20%
Partnership Interest [Member] | Term Loan Agreement [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 500,000	1,000,000
Repayment of Senior Notes		$ 500,000					$ 500,000
Delayed-Draw Unsecured Senior Term Loan					$ 1,000,000
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate | Rate				2.87%	2.04%
Partnership Interest [Member] | Notes Payables due 2019 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 500,000	$ 500,000
Interest rates (in hundredths) | Rate				8.625%
Partnership Interest [Member] | Note Payable from Public Offering Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 300,000	300,000
Interest rates (in hundredths) | Rate				3.80%
Partnership Interest [Member] | Note Payable 2 from Public Offering Due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 900,000	900,000
Interest rates (in hundredths) | Rate				3.375%
Partnership Interest [Member] | Note Payable from Public Offering Due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 425,000	425,000
Interest rates (in hundredths) | Rate				5.00%
Partnership Interest [Member] | Note Payable from Public Offering Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 500,000	500,000
Interest rates (in hundredths) | Rate				4.90%
Partnership Interest [Member] | Notes Payables due 2036 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 600,000	600,000
Interest rates (in hundredths) | Rate				6.65%
Partnership Interest [Member] | Notes Payables due 2037 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 600,000	600,000
Interest rates (in hundredths) | Rate				6.85%
Partnership Interest [Member] | Note Payable from Public Offering Due 2041 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 650,000	650,000
Interest rates (in hundredths) | Rate				6.125%
Partnership Interest [Member] | Note Payable from Public Offering Due 2043 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 400,000	400,000
Interest rates (in hundredths) | Rate				6.20%
Partnership Interest [Member] | Notes Payable due 2016 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 450,000
Interest rates (in hundredths) | Rate					6.15%
Natural Gas Gathering And Processing and Natural Gas Liquids [Member]
Debt Instrument [Line Items]
Impairment of long-lived assets						$ 20,200