DEBT (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) Rate	Dec. 31, 2018 USD ($) Rate	Dec. 31, 2017 USD ($) Rate	Dec. 31, 2016 USD ($) Rate	Sep. 30, 2018 USD ($) Rate
Debt Instrument [Line Items]
Impairment of long-lived assets		$ 0	$ 15,970	$ 0
Total debt		9,451,354	9,198,677
Unamortized portion of terminated swaps		16,750	18,468
Unamortized debt issuance costs and discounts		(87,120)	(78,193)
Current maturities of long-term debt		(507,650)	(432,650)
Short-term borrowings		0	(614,673)
Long-term debt		8,873,334	8,091,629
Senior Notes, Noncurrent	$ 1,200,000	1,250,000
Long-term Debt, by Maturity [Abstract]
2019		507,700
2020		307,700
2021		557,700
2022		1,453,300
2023		925,000
$2.5 Billion Credit Agreement [Member]
Debt Instrument [Line Items]
Letters of Credit Outstanding, Amount		1,400	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		3.5
$2.5 Billion Credit Agreement [Member]
Debt Instrument [Line Items]
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). At December 31, 2018, due to our acquisition of the remaining 20 percent interest in WTLPG for $195 million, the covenant increased to 5.5 to 1 for the second half of 2018 and first quarter 2019, and 5.0 to 1 thereafter. Our $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 our $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. Our $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 an additional one-year extension, 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, 2018, our ratio of indebtedness to adjusted EBITDA was 3.5 to 1, and we were in compliance with all covenants under our $2.5 Billion Credit Agreement.
Senior Notes [Member]
Debt Instrument [Line Items]
Senior Notes, Noncurrent					$ 1,250,000
Proceeds from Debt, Net of Issuance Costs	1,180,000	$ 1,230,000
Note Payable from Public Offering Due 2018 [Member]
Long-term Debt, by Maturity [Abstract]
2019		500,000
Notes Payables due 2019 [Member]
Long-term Debt, by Maturity [Abstract]
2020		300,000
Note Payable from Public Offering Due 2020 [Member]
Long-term Debt, by Maturity [Abstract]
2021		550,000
Note Payable Due 2022 [Domain]
Long-term Debt, by Maturity [Abstract]
2022		1,447,400
Note Payable 2 from Public Offering Due 2022 [Member]
Long-term Debt, by Maturity [Abstract]
2023		$ 925,000
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, 2018, Guardian Pipeline was in compliance with its financial covenants
Guardian Pipeline [Member] | Notes Payables 1 due 2022 [Member]
Debt Instrument [Line Items]
Total debt		$ 28,957	36,607
Average interest rate (in hundredths) | Rate		7.85%
Long-term Debt, by Maturity [Abstract]
2019		$ 7,700
2020		7,700
2021		7,700
2022		5,900
2023		0
Subsidiary Issuer [Member] | Notes Payable due 2016 [Member]
Debt Instrument [Line Items]
Total debt				$ 450,000
Interest rates (in hundredths) | Rate				6.15%
Subsidiary Issuer [Member] | Note Payable from Public Offering Due 2017 [Member]
Debt Instrument [Line Items]
Repayment of Senior Notes	$ 400,000
Interest rates (in hundredths) | Rate	2.00%
Subsidiary Issuer [Member] | Note Payable from Public Offering Due 2018 [Member]
Debt Instrument [Line Items]
Total debt		$ 0	425,000
Repayment of Senior Notes					$ 425,000
Interest rates (in hundredths) | Rate		3.20%			3.20%
Subsidiary Issuer [Member] | Notes Payables due 2019 [Member]
Debt Instrument [Line Items]
Total debt		$ 500,000	500,000
Interest rates (in hundredths) | Rate		8.625%
Subsidiary Issuer [Member] | Note Payable from Public Offering Due 2020 [Member]
Debt Instrument [Line Items]
Total debt		$ 300,000	300,000
Interest rates (in hundredths) | Rate		3.80%
Subsidiary Issuer [Member] | Term Loan Agreement due 2019 [Member]
Debt Instrument [Line Items]
Total debt		$ 0	$ 500,000
Repayment of Senior Notes	$ 500,000
Delayed-Draw Unsecured Senior Term Loan				$ 1,000,000
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate | Rate			2.87%
Subsidiary Issuer [Member] | Note Payable 2 from Public Offering Due 2022 [Member]
Debt Instrument [Line Items]
Total debt		$ 900,000	$ 900,000
Interest rates (in hundredths) | Rate		3.375%
Subsidiary Issuer [Member] | Note Payable from Public Offering Due 2023 [Member]
Debt Instrument [Line Items]
Total debt		$ 425,000	425,000
Interest rates (in hundredths) | Rate		5.00%
Subsidiary Issuer [Member] | Note Payable from Public Offering Due 2025 [Member]
Debt Instrument [Line Items]
Total debt		$ 500,000	500,000
Interest rates (in hundredths) | Rate		4.90%
Subsidiary Issuer [Member] | Notes Payables due 2036 [Member]
Debt Instrument [Line Items]
Total debt		$ 600,000	600,000
Interest rates (in hundredths) | Rate		6.65%
Subsidiary Issuer [Member] | Notes Payables due 2037 [Member]
Debt Instrument [Line Items]
Total debt		$ 600,000	600,000
Interest rates (in hundredths) | Rate		6.85%
Subsidiary Issuer [Member] | Note Payable from Public Offering Due 2041 [Member]
Debt Instrument [Line Items]
Total debt		$ 650,000	650,000
Interest rates (in hundredths) | Rate		6.125%
Subsidiary Issuer [Member] | Note Payable from Public Offering Due 2043 [Member]
Debt Instrument [Line Items]
Total debt		$ 400,000	400,000
Interest rates (in hundredths) | Rate		6.20%
Parent Company
Debt Instrument [Line Items]
Commercial Paper		$ 0	$ 614,673
Short-term Debt, Weighted Average Interest Rate, at Point in Time | Rate		2.85%	2.23%
Parent Company | Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument covenant description		Our senior notes are governed by indentures containing covenants, including among other provisions, limitations on our ability to place liens on our property or assets and to sell and leaseback our property. The indentures governing our 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 remainder of our senior notes 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 to declare those senior notes immediately due and payable in full. The indenture for the 7.5 percent 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		We may redeem our 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. We may redeem the balance of our senior notes due 2020, 2022, 2023, 2025, 2027, 2028 (4.55%), 2041, 2043, 2047 and 2048 at a redemption price equal to the principal amount, plus accrued and unpaid interest, starting one to six months before the maturity date as stipulated in the respective contract terms. Our senior notes are senior unsecured obligations, ranking equally in right of payment with all of our existing and future unsecured senior indebtedness.
Parent Company | Term Loan Agreement due 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate Terms		bears interest at LIBOR plus 112.5 basis points
Total debt		$ 550,000	$ 0
Parent Company | Note Payable from Public Offering Due 2022 [Member]
Debt Instrument [Line Items]
Total debt		$ 547,397	547,397
Interest rates (in hundredths) | Rate		4.25%
Parent Company | Note Payable Due 2023 [Member]
Debt Instrument [Line Items]
Total debt		$ 500,000	500,000
Interest rates (in hundredths) | Rate		7.50%
Parent Company | Note Payable Due 2027 [Member]
Debt Instrument [Line Items]
Total debt	$ 500,000	$ 500,000	500,000
Interest rates (in hundredths) | Rate	4.00%	4.00%
Parent Company | Note Payables 1 due 2028 [Member]
Debt Instrument [Line Items]
Total debt		$ 800,000	0
Repayment of Senior Notes	$ 87,000
Interest rates (in hundredths) | Rate	6.50%	4.55%
Parent Company | Note Payables 2 due 2028 [Member]
Debt Instrument [Line Items]
Total debt		$ 100,000	100,000
Interest rates (in hundredths) | Rate		6.875%
Parent Company | Note Payable from Public Offering Due 2028 [Member]
Debt Instrument [Line Items]
Total debt					$ 800,000
Interest rates (in hundredths) | Rate					4.55%
Parent Company | Notes Payables due 2035 [Member]
Debt Instrument [Line Items]
Total debt		$ 400,000	400,000
Interest rates (in hundredths) | Rate		6.00%
Parent Company | Notes Payables due 2047 [Member]
Debt Instrument [Line Items]
Total debt	$ 700,000	$ 700,000	700,000
Interest rates (in hundredths) | Rate	4.95%	4.95%
Parent Company | Note Payable from Public Offering Due 2047 [Member]
Debt Instrument [Line Items]
Total debt					$ 450,000
Interest rates (in hundredths) | Rate					5.20%
Parent Company | Note Payables due 2048 [Member]
Debt Instrument [Line Items]
Total debt		$ 450,000	$ 0
Interest rates (in hundredths) | Rate		5.20%
Parent Company | Term Loan Agreement due 2019 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate | Rate		3.63%