INCOME TAXES (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income tax provision
Federal			$ 295,000	$ 6,086,000	$ 13,191,000
State			1,670,000	2,449,000	2,967,000
Total current income taxes from continuing operations			1,965,000	8,535,000	16,158,000
Deferred income tax provision
Federal			376,728,000	193,974,000	116,681,000
State			68,589,000	9,897,000	3,761,000
Total deferred income taxes from continuing operations			445,317,000	203,871,000	120,442,000
Total provision for income taxes from continuing operations			447,282,000	212,406,000	136,600,000
Discontinued operations			0	(1,250,000)	2,031,000
Total provision for income taxes			447,282,000	211,156,000	138,631,000
Income Tax Reconciliation [Abstract]
Income before income taxes			1,040,801,000	957,956,000	521,876,000
Less: Net income attributable to noncontrolling interests			205,678,000	391,460,000	134,218,000
Net income attributable to ONEOK before income taxes			$ 835,123,000	$ 566,496,000	$ 387,658,000
Federal statutory income tax rate		21.00%	35.00%	35.00%	35.00%
Provision for federal income taxes			$ 292,293,000	$ 198,274,000	$ 135,680,000
State income taxes, net of federal benefit			16,197,000	12,303,000	5,800,000
Deferred tax rate change, inclusive of valuation allowance			141,283,000	43,000	928,000
Other, net			(2,491,000)	1,786,000	$ (5,808,000)
Deferred tax assets
Employee benefits and other accrued liabilities			85,355,000	118,831,000
Federal net operating loss			159,162,000	26,334,000
State net operating loss and benefits			73,277,000	39,759,000
Derivative instruments			30,060,000	32,082,000
Other			13,546,000	2,425,000
Total deferred tax assets			361,400,000	219,431,000
Carryforward expected to expire prior to utilization			(66,632,000)	(9,430,000)
Net deferred tax assets			294,768,000	210,001,000
Deferred tax liabilities
Excess of tax over book depreciation			64,508,000	107,249,000
Investment in partnerships			77,035,000	1,726,541,000
Regulatory assets			15,000	33,000
Total deferred tax liabilities			141,558,000	1,833,823,000
Net deferred tax assets (liabilities) before discontinued operations			153,210,000	(1,623,822,000)
Discontinued operations			0	10,500,000
Net deferred tax assets (liabilities)			153,210,000	$ (1,613,322,000)
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount			54.1
Deferred Tax Assets, Gross, Noncurrent	$ 2,100,000,000
Deferred Tax Asset, Tax Basis	8,800,000,000
Deferred Tax Asset, Book Basis	$ 3,000,000,000
Excess Tax Benefit from Share-based Compensation, Operating Activities			$ 73,400,000
ONEOK
Income Tax Reconciliation [Abstract]
Federal statutory income tax rate	37.00%