SUPPLEMENTAL CONDENSED CONSOLIDATING FINANCIAL INFORMATION CONDENSED CONSOLIDATING FINANCIAL INFORMATION, Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net income	$ 292,888	$ 313,916	$ 282,179	$ 266,049	$ 64,812	$ 166,531	$ 175,991	$ 186,185	$ 1,155,032	$ 593,519	$ 743,499
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									(5,673)	(21,408)	(30,300)
Realized (gains) losses on derivatives recognized in net income, net of tax									(36,870)	(63,687)	6,977
Change in pension and postretirement benefit plan liability, net of tax									(4,771)	4,175	16,693
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax									2,424	(970)	(1,505)
Total other comprehensive income (loss), net of tax									38,392	37,134	(55,475)
Comprehensive income									1,193,424	630,653	688,024
Less: Comprehensive income attributable to noncontrolling interests									3,329	236,704	363,093
Comprehensive income attributable to ONEOK									1,190,095	393,949	324,931
Reportable Legal Entities | Parent Issuer & Guarantor
Condensed Financial Statements, Captions [Line Items]
Net income									1,151,700	589,200	738,000
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									(46,700)	19,100	0
Realized (gains) losses on derivatives recognized in net income, net of tax									1,900	2,500	2,100
Change in pension and postretirement benefit plan liability, net of tax									5,300	(4,200)	(16,700)
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax									0	0	0
Total other comprehensive income (loss), net of tax									(39,500)	17,400	(14,600)
Comprehensive income									1,112,200	606,600	723,400
Less: Comprehensive income attributable to noncontrolling interests									0	232,400	357,600
Comprehensive income attributable to ONEOK									1,112,200	374,200	365,800
Reportable Legal Entities | Subsidiary Issuer & Guarantor
Condensed Financial Statements, Captions [Line Items]
Net income									1,660,500	1,215,700	1,066,800
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									53,200	(72,200)	(35,800)
Realized (gains) losses on derivatives recognized in net income, net of tax									45,500	86,500	(10,700)
Change in pension and postretirement benefit plan liability, net of tax									(700)	0	0
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax									3,100	(1,100)	(1,800)
Total other comprehensive income (loss), net of tax									101,100	13,200	(48,300)
Comprehensive income									1,761,600	1,228,900	1,018,500
Less: Comprehensive income attributable to noncontrolling interests									0	0	0
Comprehensive income attributable to ONEOK									1,761,600	1,228,900	1,018,500
Reportable Legal Entities | Guarantor Subsidiary
Condensed Financial Statements, Captions [Line Items]
Net income									1,660,500	1,215,700	1,066,800
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									53,200	(40,600)	(78,500)
Realized (gains) losses on derivatives recognized in net income, net of tax									29,600	69,600	(26,400)
Change in pension and postretirement benefit plan liability, net of tax									0	0	0
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax									3,100	(1,100)	(1,800)
Total other comprehensive income (loss), net of tax									85,900	27,900	(106,700)
Comprehensive income									1,746,400	1,243,600	960,100
Less: Comprehensive income attributable to noncontrolling interests									0	0	0
Comprehensive income attributable to ONEOK									1,746,400	1,243,600	960,100
Reportable Legal Entities | Combined Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income									1,616,800	1,191,500	999,400
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									41,000	(8,800)	(108,800)
Realized (gains) losses on derivatives recognized in net income, net of tax									19,100	44,300	(33,400)
Change in pension and postretirement benefit plan liability, net of tax									200	0	0
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax									2,300	(1,000)	(3,300)
Total other comprehensive income (loss), net of tax									62,600	34,500	(145,500)
Comprehensive income									1,679,400	1,226,000	853,900
Less: Comprehensive income attributable to noncontrolling interests									3,300	4,300	5,500
Comprehensive income attributable to ONEOK									1,676,100	1,221,700	848,400
Reportable Legal Entities | Total
Condensed Financial Statements, Captions [Line Items]
Net income									1,155,000	593,500	743,500
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									(5,700)	(21,400)	(30,300)
Realized (gains) losses on derivatives recognized in net income, net of tax									36,900	63,700	(7,000)
Change in pension and postretirement benefit plan liability, net of tax									4,800	(4,200)	(16,700)
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax									2,400	(1,000)	(1,500)
Total other comprehensive income (loss), net of tax									38,400	37,100	(55,500)
Comprehensive income									1,193,400	630,600	688,000
Less: Comprehensive income attributable to noncontrolling interests									3,300	236,700	363,100
Comprehensive income attributable to ONEOK									1,190,100	393,900	324,900
Consolidating Entries
Condensed Financial Statements, Captions [Line Items]
Net income									(4,934,500)	(3,618,600)	(3,127,500)
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									(106,400)	81,100	192,800
Realized (gains) losses on derivatives recognized in net income, net of tax									(59,200)	(139,200)	61,400
Change in pension and postretirement benefit plan liability, net of tax									0	0	0
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax									(6,100)	2,200	5,400
Total other comprehensive income (loss), net of tax									(171,700)	(55,900)	259,600
Comprehensive income									(5,106,200)	(3,674,500)	(2,867,900)
Less: Comprehensive income attributable to noncontrolling interests									0	0	0
Comprehensive income attributable to ONEOK									$ (5,106,200)	$ (3,674,500)	$ (2,867,900)