ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrealized gains (losses) on risk management assets/liabilities - January 1	$ (52,155)	$ (42,199)
Pension and Postretirement Benefit Plan Obligations - January 1	(101,236)	(84,543)
Unrealized gains (losses) on risk-management assets/liabilities of unconsolidated affiliates - January 1	(959)	(500)
Accumulated other comprehensive loss - January 1	(154,350)	(127,242)
Other comprehensive income (loss), unrealized gain (loss) on derivatives arising during period, net of tax	(35,013)	(9,280)
Other comprehensive (income) loss, defined benefit plan, before reclassification adjustment, after tax	(12,337)	(22,903)
Other comprehensive income (loss), amount attributable to equity method investments, before reclassification adjustments, net of tax	(409)	(475)
Other comprehensive income (loss) before reclassifications	(47,759)	(32,658)
Realized (gains) losses on derivatives in net income, net of tax	63,687	(6,977)	$ (54,709)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Other Comprehensive Income Attributable to Equity Method Investments, Net of Tax	164	16
Other Comprehensive Income (Loss), Reclassification Adjustment included in Net Income, Net of Tax	53,867	5,550	(5,600)
Impact of Merger Transaction On Unrealized Gains (Losses) on Risk Management Assets/Liabilities	(40,288)
Impact of Merger Transaction on Pension and Postretirement Benefit Plan Obligations	0
Impact of Merger Transaction on Unrealized Gains (Losses) on Risk Management Assets/Liabilities of Unconsolidated Affiliates	0
Impact of Merger Transaction on AOCI, Total	(40,288)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax, Portion Attributable to Parent	(29,760)	(9,956)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	(4,175)	(16,693)	15,416
Other Comprehensive Income (Loss) on Investment in Unconsolidated Affiliates	(245)	(459)
Net Current-Period Other Comprehensive Income (Loss) Attributable to ONEOK	(34,180)	(27,108)
Unrealized gains (losses) on risk management assets/liabilities - December 31	(81,915)	(52,155)	(42,199)
Pension and Postretirement Benefit Plan Obligations - December 31	(105,411)	(101,236)	(84,543)
Unrealized gains (losses) on risk-management assets/liabilities of unconsolidated affiliates - December 31	(1,204)	(959)	(500)
Accumulated other comprehensive loss - December 31	(188,530)	(154,350)	(127,242)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	26,899	(230)	(8,815)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Realized (gains) losses on derivatives in net income, net of tax	45,541	(676)	(15,294)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, after Tax	(8,162)	(6,210)	(9,694)
Defined Benefit Plan, Amortization of Gain (Loss)	(15,265)	(12,012)	(17,724)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	1,662	1,662	1,568
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	(13,603)	(10,350)	(16,156)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, Tax	5,441	4,140	6,462
Accumulated Other Comprehensive Income from Investments in Unconsolidated Affiliates Attributable to Parent [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Other Comprehensive Income Attributable to Equity Method Investments, Net of Tax	164	16	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Other Comprehensive Income Attributable to Equity Method Investments, Before Tax	(367)	(63)	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Other Comprehensive Income Attributable to Equity Method Investments, Tax	97	10	0
Sales [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Commodity Contract [Member]
Amounts reclassified from accumulated other comprehensive income (loss)	(69,561)	26,422	81,089
Interest Expense [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Interest Rate Contracts
Amounts reclassified from accumulated other comprehensive income (loss)	(21,025)	(19,215)	(17,565)
Total before tax [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Amounts reclassified from accumulated other comprehensive income (loss)	(90,586)	7,207	63,524
Tax Expense [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	26,899	(230)	(8,815)
Net of tax [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Realized (gains) losses on derivatives in net income, net of tax	63,687	(6,977)	(54,709)
Net of tax [Member] | Accumulated Other Comprehensive Income from Investments in Unconsolidated Affiliates Attributable to Parent [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Other Comprehensive Income Attributable to Equity Method Investments, Net of Tax	270	53	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Other Comprehensive Income Attributable to Equity Method Investments, Net of Tax, Portion Attributable to Noncont Interests	(106)	(37)	0
Net income attributable to noncontrolling interest [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax, Portion Attributable to Noncontrolling Interest	$ (18,146)	$ 6,301	$ 39,415