SUPPLEMENTAL CONDENSED CONSOLIDATING FINANCIAL INFORMATION CONDENSED CONSOLIDATING FINANCIAL INFORMATION, Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net income	$ 64,812	$ 166,531	$ 175,991	$ 186,185	$ 194,465	$ 194,216	$ 179,859	$ 174,959	$ 593,519	$ 743,499	$ 379,195
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									(21,408)	(30,300)	41,362
Realized (gains) losses on derivatives in net income, net of tax									(63,687)	6,977	54,709
Unrealized holding gains (losses) on available-for-sale securities, net of tax									0	0	(955)
Change in pension and postretirement benefit plan liability, net of tax									4,175	16,693	(15,416)
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax									(970)	(1,505)	(1,632)
Total other comprehensive income (loss), net of tax									37,134	(55,475)	(518)
Comprehensive income									630,653	688,024	378,677
Less: Comprehensive income attributable to noncontrolling interests									236,704	363,093	124,589
Comprehensive income attributable to ONEOK									393,949	324,931	254,088
Reportable Legal Entities | Parent Issuer & Guarantor
Condensed Financial Statements, Captions [Line Items]
Net income									589,200	738,000	370,800
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									19,100	0	0
Realized (gains) losses on derivatives in net income, net of tax									2,500	2,100	2,100
Unrealized holding gains (losses) on available-for-sale securities, net of tax											0
Change in pension and postretirement benefit plan liability, net of tax									(4,200)	(16,700)	15,400
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax									0	0	0
Total other comprehensive income (loss), net of tax									17,400	(14,600)	17,500
Comprehensive income									606,600	723,400	388,300
Less: Comprehensive income attributable to noncontrolling interests									232,400	357,600	116,200
Comprehensive income attributable to ONEOK									374,200	365,800	272,100
Reportable Legal Entities | Subsidiary Issuer & Guarantor
Condensed Financial Statements, Captions [Line Items]
Net income									1,215,700	1,066,800	589,500
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									(72,200)	(35,800)	47,500
Realized (gains) losses on derivatives in net income, net of tax									86,500	(10,700)	(67,000)
Unrealized holding gains (losses) on available-for-sale securities, net of tax											0
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									(1,100)	(1,800)	(1,900)
Total other comprehensive income (loss), net of tax									13,200	(48,300)	(21,400)
Comprehensive income									1,228,900	1,018,500	568,100
Less: Comprehensive income attributable to noncontrolling interests									0	0	0
Comprehensive income attributable to ONEOK									1,228,900	1,018,500	568,100
Reportable Legal Entities | Guarantor Subsidiary
Condensed Financial Statements, Captions [Line Items]
Net income									1,215,700	1,066,800	589,500
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									(40,600)	(78,500)	70,100
Realized (gains) losses on derivatives in net income, net of tax									69,600	(26,400)	(81,100)
Unrealized holding gains (losses) on available-for-sale securities, net of tax											0
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									(1,100)	(1,800)	(1,900)
Total other comprehensive income (loss), net of tax									27,900	(106,700)	(12,900)
Comprehensive income									1,243,600	960,100	576,600
Less: Comprehensive income attributable to noncontrolling interests									0	0	0
Comprehensive income attributable to ONEOK									1,243,600	960,100	576,600
Reportable Legal Entities | Combined Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income									1,191,500	999,400	525,300
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									(8,800)	(108,800)	111,500
Realized (gains) losses on derivatives in net income, net of tax									44,300	(33,400)	(137,900)
Unrealized holding gains (losses) on available-for-sale securities, net of tax											(1,000)
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									(1,000)	(3,300)	(3,500)
Total other comprehensive income (loss), net of tax									34,500	(145,500)	(30,900)
Comprehensive income									1,226,000	853,900	494,400
Less: Comprehensive income attributable to noncontrolling interests									4,300	5,500	8,400
Comprehensive income attributable to ONEOK									1,221,700	848,400	486,000
Reportable Legal Entities | Total
Condensed Financial Statements, Captions [Line Items]
Net income									593,500	743,500	379,200
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									(21,400)	(30,300)	41,400
Realized (gains) losses on derivatives in net income, net of tax									63,700	(7,000)	(54,700)
Unrealized holding gains (losses) on available-for-sale securities, net of tax											(1,000)
Change in pension and postretirement benefit plan liability, net of tax									(4,200)	(16,700)	15,400
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax									(1,000)	(1,500)	(1,600)
Total other comprehensive income (loss), net of tax									37,100	(55,500)	(500)
Comprehensive income									630,600	688,000	378,700
Less: Comprehensive income attributable to noncontrolling interests									236,700	363,100	124,600
Comprehensive income attributable to ONEOK									393,900	324,900	254,100
Consolidating Entries
Condensed Financial Statements, Captions [Line Items]
Net income									(3,618,600)	(3,127,500)	(1,695,900)
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax									81,100	192,800	(187,700)
Realized (gains) losses on derivatives in net income, net of tax									(139,200)	61,400	229,200
Unrealized holding gains (losses) on available-for-sale securities, net of tax											0
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									2,200	5,400	5,700
Total other comprehensive income (loss), net of tax									(55,900)	259,600	47,200
Comprehensive income									(3,674,500)	(2,867,900)	(1,648,700)
Less: Comprehensive income attributable to noncontrolling interests									0	0	0
Comprehensive income attributable to ONEOK									$ (3,674,500)	$ (2,867,900)	$ (1,648,700)