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DEBT (Details) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Mar. 31, 2019 USD ($) Rate | Sep. 30, 2019 USD ($) Rate | Sep. 30, 2018 USD ($) Rate | Sep. 30, 2017 USD ($) Rate | Dec. 31, 2019 USD ($) Rate | Dec. 31, 2018 USD ($) Rate
Debt Instrument [Line Items] |  |  |  |  |  |
Total debt |  |  |  |  | $ 12,813,704 | $ 9,451,354
Unamortized portion of terminated swaps |  |  |  |  | (15,032) | (16,750)
Unamortized debt issuance costs and discounts |  |  |  |  | (121,329) | (87,120)
Current maturities of long-term debt |  |  |  |  | (7,650) | (507,650)
Short-term borrowings |  |  |  |  | (220,000) | 0
Long-term debt |  |  |  |  | 12,479,757 | 8,873,334
Letters of Credit Outstanding, Amount |  |  |  |  | 4,700 | 1,400
Senior Note Covenant, Acceleration Of Indebtedness |  |  |  |  | $ 100,000 |
Senior Note Covenant, Trustee Or Holders Percentage Required Upon Event Of Default |  |  |  |  | 25.00% |
Long-term Debt, by Maturity [Abstract] |  |  |  |  |  |
2020 |  |  |  |  | $ 7,700 |
2021 |  |  |  |  | 1,257,700 |
2022 |  |  |  |  | 1,453,300 |
2023 |  |  |  |  | 925,000 |
2024 |  |  |  |  | 500,000 |
$2.5 Billion Credit Agreement [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  |  |  | 2,500,000 |
Line of Credit Facility, Amount Outstanding |  |  |  |  | $ 0 | 0
Indebtedness to Adjusted EBITDA Maximum |  |  |  |  | 5.0 |
Indebtedness To Adjusted EBITDA From Acquisitions Maximum |  |  |  |  | 5.5 |
Aggregate Purchase Price For Acquisitions Threshold Which Adjusts Allowable Ratio Of Indebtedness To Adjusted EBITDA |  |  |  |  | $ 25,000 |
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, Annual Facility Fee |  |  |  |  | 0.15% |
Indebtedness To Adjusted EBITDA Current |  |  |  |  | 4.1 |
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 5.0 to 1 at December 31, 2019. 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 following two quarters. Thereafter, the covenant will decrease to 5.0 to 1. 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. At December 31, 2019, our ratio of indebtedness to adjusted EBITDA was 4.1 to 1, and we were in compliance with all covenants under our $2.5 Billion Credit Agreement. |
Notes Payable from Public Offering Due 2024, 2029 and 2049 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Senior Notes, Noncurrent |  | $ 2,000,000 |  |  | $ 2,000,000 |
Proceeds from Debt, Net of Issuance Costs |  | 1,970,000 |  |  |  |
Notes Payable from Public Offering Due 2029 and 2048 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Senior Notes, Noncurrent | $ 1,250,000 |  |  |  | 1,250,000 |
Proceeds from Debt, Net of Issuance Costs | 1,230,000 |  |  |  |  |
Notes Payable from Public Offering Due 2028 and 2048 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Senior Notes, Noncurrent |  |  | $ 1,250,000 |  |  |
Proceeds from Debt, Net of Issuance Costs |  |  | 1,230,000 |  |  |
Notes Payable from Public Offering Due 2027 and 2047 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Senior Notes, Noncurrent |  |  |  | $ 1,200,000 |  |
Proceeds from Debt, Net of Issuance Costs |  |  |  | 1,200,000 |  |
Senior Notes [Member] |  |  |  |  |  |
Long-term Debt, by Maturity [Abstract] |  |  |  |  |  |
2020 |  |  |  |  | 0 |
2021 |  |  |  |  | 1,250,000 |
2022 |  |  |  |  | 1,447,400 |
2023 |  |  |  |  | 925,000 |
2024 |  |  |  |  | $ 500,000 |
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, 2019, Guardian Pipeline was in compliance with its financial covenants |
Guardian Pipeline [Member] | Notes Payables 1 due 2022 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total debt |  |  |  |  | $ 21,307 | 28,957
Average interest rate (in hundredths) | Rate |  |  |  |  | 7.85% |
Long-term Debt, by Maturity [Abstract] |  |  |  |  |  |
2020 |  |  |  |  | $ 7,700 |
2021 |  |  |  |  | 7,700 |
2022 |  |  |  |  | 5,900 |
2023 |  |  |  |  | 0 |
2024 |  |  |  |  | 0 |
Subsidiary Issuer [Member] | Note Payable from Public Offering Due 2017 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Repayments of Long-term Debt |  |  |  | $ 400,000 |  |
Interest rates (in hundredths) | Rate |  |  |  | 2.00% |  |
Subsidiary Issuer [Member] | Term Loan Agreement due 2019 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Repayments of Long-term Debt |  |  | 500,000 | $ 500,000 |  |
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% |
Subsidiary Issuer [Member] | Notes Payables due 2019 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total debt |  |  |  |  | $ 0 | 500,000
Repayments of Long-term Debt | $ 500,000 |  |  |  |  |
Interest rates (in hundredths) | Rate | 8.625% |  |  |  | 8.625% |
Subsidiary Issuer [Member] | Note Payable from Public Offering Due 2018 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Repayments of Long-term Debt |  |  | $ 425,000 |  |  |
Interest rates (in hundredths) | Rate |  |  | 3.20% |  |  |
Subsidiary Issuer [Member] | Note Payable from Public Offering Due 2020 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total debt |  |  |  |  | $ 0 | 300,000
Extinguishment of Debt, Amount |  | $ 300,000 |  |  |  |
Interest rates (in hundredths) | Rate |  | 3.80% |  |  | 3.80% |
Long-term Debt, by Maturity [Abstract] |  |  |  |  |  |
Debt Instrument, Redemption Price |  | $ 308,000 |  |  |  |
Gain (Loss) on Extinguishment of Debt |  | (2,700) |  |  |  |
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% |
Parent Company |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Commercial Paper |  |  |  |  | $ 220,000 | $ 0
Short-term Debt, Weighted Average Interest Rate, at Point in Time | Rate |  |  |  |  | 2.16% | 0.00%
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% 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% 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% 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% 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 2022, 2023, 2024, 2025, 2027, 2028 (4.55%), 2029, 2041, 2043, 2047, 2048 and 2049 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] |  |  |  |  |  |
Total debt |  |  |  |  | $ 1,250,000 | $ 550,000
Debt Instrument, Face Amount |  |  |  |  | 1,500,000 |
Repayments of Long-term Debt |  | 250,000 |  |  |  |
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% |
Senior Note Covenant, Redemption Price Upon Event Of Default | Rate |  |  |  |  | 101.00% |
Parent Company | Note Payable Due 2024 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total debt |  | $ 500,000 |  |  | $ 500,000 | 0
Interest rates (in hundredths) | Rate |  | 2.75% |  |  | 2.75% |
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 |  | $ 800,000 | 800,000
Interest rates (in hundredths) | Rate |  |  | 4.55% |  | 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% |
Senior Note Covenant, Acceleration Of Indebtedness |  |  |  |  | $ 15,000 |
Parent Company | Note Payable from Public Offering Due 2029 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total debt | $ 700,000 |  |  |  | $ 700,000 | 0
Interest rates (in hundredths) | Rate | 4.35% |  |  |  | 4.35% |
Parent Company | Note Payables, Additional Issuance, due 2048 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total debt | $ 550,000 |  |  |  |  |
Interest rates (in hundredths) | Rate | 5.20% |  |  |  |  |
Parent Company | Note Payable Due 2029 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total debt |  | $ 750,000 |  |  | $ 750,000 | 0
Interest rates (in hundredths) | Rate |  | 3.40% |  |  | 3.40% |
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] |  |  |  |  |  |
Interest rates (in hundredths) | Rate |  |  | 5.20% |  |  |
Parent Company | Note Payables due 2048 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total debt |  |  | $ 450,000 |  | $ 1,000,000 | 450,000
Interest rates (in hundredths) | Rate |  |  |  |  | 5.20% |
Parent Company | Note Payable Due 2049 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total debt |  | $ 750,000 |  |  | $ 750,000 | $ 0
Interest rates (in hundredths) | Rate |  | 4.45% |  |  | 4.45% |
Parent Company | Term Loan Agreement due 2019 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate | Rate |  |  |  |  | 2.70% | 3.63%
Parent Company | Note Payable from Public Offering Due 2028 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Interest rates (in hundredths) | Rate |  |  |  | 6.50% |  |
Long-term Debt, by Maturity [Abstract] |  |  |  |  |  |
Debt Instrument, Redemption Price |  |  |  | $ 87,000 |  |
London Interbank Offered Rate (LIBOR) [Member] | $2.5 Billion Credit Agreement [Member] |  |  |  |  |  |
Long-term Debt, by Maturity [Abstract] |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  | 1.10% |
London Interbank Offered Rate (LIBOR) [Member] | Parent Company | Term Loan Agreement due 2021 [Member] |  |  |  |  |  |
Long-term Debt, by Maturity [Abstract] |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  | 1.125% |

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