SUPPLEMENTAL CONDENSED CONSOLIDATING FINANCIAL INFORMATION (Tables)

12 Months Ended

Dec. 31, 2018

Condensed Financial Information of Parent Company Only Disclosure [Abstract]

Condensed Consolidating Statements of Income

	Year Ended December 31, 2018
	Parent Issuer & Guarantor			Subsidiary Issuer & Guarantor			Guarantor Subsidiary			Combined Non-Guarantor Subsidiaries			Consolidating Entries			Total
	(Millions of dollars)
Revenues
Commodity sales	$	—		$	—		$	—		$	11,395.6		$	—		$	11,395.6
Services	—			—			—			1,199.7			(2.1		)	1,197.6
Total revenues	—			—			—			12,595.3			(2.1		)	12,593.2
Cost of sales and fuel (exclusive of items shown separately below)	—			—			—			9,422.7			—			9,422.7
Operating expenses	(0.6		)	—			—			1,338.3			(2.1		)	1,335.6
Gain on sale of assets	—			—			—			(0.6		)	—			(0.6		)
Operating income	0.6			—			—			1,834.9			—			1,835.5
Equity in net earnings from investments	1,655.6			1,660.5			1,660.5			116.3			(4,934.5		)	158.4
Other income (expense), net	29.6			315.1			315.1			(36.0		)	(630.2		)	(6.4		)
Interest expense, net	(179.4		)	(315.1		)	(315.1		)	(290.2		)	630.2			(469.6		)
Income before income taxes	1,506.4			1,660.5			1,660.5			1,625.0			(4,934.5		)	1,517.9
Income taxes	(354.7		)	—			—			(8.2		)	—			(362.9		)
Net income	1,151.7			1,660.5			1,660.5			1,616.8			(4,934.5		)	1,155.0
Less: Net income attributable to noncontrolling interests	—			—			—			3.3			—			3.3
Net income attributable to ONEOK	1,151.7			1,660.5			1,660.5			1,613.5			(4,934.5		)	1,151.7
Less: Preferred stock dividends	1.1			—			—			—			—			1.1
Net income available to common shareholders	$	1,150.6		$	1,660.5		$	1,660.5		$	1,613.5		$	(4,934.5	)	$	1,150.6

	Year Ending December 31, 2017
	Parent Issuer & Guarantor			Subsidiary Issuer & Guarantor			Guarantor Subsidiary				Combined Non-Guarantor Subsidiaries			Consolidating Entries			Total
	(Millions of dollars)
Revenues
Commodity sales	$	—		$	—		$	—			$	9,862.7		$	—		$	9,862.7
Services	—			—			—				2,313.2			(2.0		)	2,311.2
Total revenues	—			—			—				12,175.9			(2.0		)	12,173.9
Cost of sales and fuel (exclusive of items shown separately below)	—			—			—				9,538.0			—			9,538.0
Operating expenses	17.8			—			9.2				1,204.0			(2.0		)	1,229.0
Impairment of long-lived assets	—			—			—				16.0			—			16.0
Gain on sale of assets	—			—			—				(0.9		)	—			(0.9		)
Operating income	(17.8		)	—			(9.2		)		1,418.8			—			1,391.8
Equity in net earnings from investments	1,236.6			1,215.7			1,224.9				100.7			(3,618.6		)	159.3
Impairment of equity investments	—			—			—				(4.3		)	—			(4.3		)
Other income (expense), net	(12.3		)	353.1			353.1				(8.0		)	(706.2		)	(20.3		)
Interest expense, net	(137.1		)	(353.1		)	(353.1		)		(348.6		)	706.2			(485.7		)
Income before income taxes	1,069.4			1,215.7			1,215.7			—	1,158.6			(3,618.6		)	1,040.8
Income taxes	(480.2		)	—			—				32.9			—			(447.3		)
Net income	589.2			1,215.7			1,215.7				1,191.5			(3,618.6		)	593.5
Less: Net income attributable to noncontrolling interests	201.4			—			—				4.3			—			205.7
Net income attributable to ONEOK	387.8			1,215.7			1,215.7				1,187.2			(3,618.6		)	387.8
Less: Preferred stock dividends	0.8			—			—				—			—			0.8
Net income available to common shareholders	$	387.0		$	1,215.7		$	1,215.7			$	1,187.2		$	(3,618.6	)	$	387.0

	Year Ending December 31, 2016
	Parent Issuer & Guarantor			Subsidiary Issuer & Guarantor			Guarantor Subsidiary			Combined Non-Guarantor Subsidiaries			Consolidating Entries			Total
	(Millions of dollars)
Revenues
Commodity sales	$	—		$	—		$	—		$	6,858.5		$	—		$	6,858.5
Services	—			—			—			2,064.3			(1.8		)	2,062.5
Total revenues	—			—			—			8,922.8			(1.8		)	8,921.0
Cost of sales and fuel (exclusive of items shown separately below)	—			—			—			6,496.1			—			6,496.1
Operating expenses	18.7			—			—			1,121.8			(1.8		)	1,138.7
(Gain) loss on sale of assets	0.3			—			—			(9.9		)	—			(9.6		)
Operating income	(19.0		)	—			—			1,314.8			—			1,295.8
Equity in net earnings from investments	1,063.9			1,066.8			1,066.8			69.7			(3,127.5		)	139.7
Other income (expense), net	(5.0		)	373.5			373.5			(2.8		)	(747.0		)	(7.8		)
Interest expense, net	(102.9		)	(373.5		)	(373.5		)	(366.8		)	747.0			(469.7		)
Income before income taxes	937.0			1,066.8			1,066.8			1,014.9			(3,127.5		)	958.0
Income taxes	(199.0		)	—			—			(13.4		)	—			(212.4		)
Income from continuing operations	738.0			1,066.8			1,066.8			1,001.5			(3,127.5		)	745.6
Income (loss) from discontinued operations, net of tax	—			—			—			(2.1		)	—			(2.1		)
Net income	738.0			1,066.8			1,066.8			999.4			(3,127.5		)	743.5
Less: Net income attributable to noncontrolling interests	386.0			—			—			5.5			—			391.5
Net income attributable to ONEOK	$	352.0		$	1,066.8		$	1,066.8		$	993.9		$	(3,127.5	)	$	352.0

Condensed Consolidating Statements of Comprehensive Income

	Year Ended December 31, 2018
	Parent Issuer & Guarantor			Subsidiary Issuer & Guarantor			Guarantor Subsidiary		Combined Non-Guarantor Subsidiaries		Consolidating Entries			Total
	(Millions of dollars)
Net income	$	1,151.7		$	1,660.5		$	1,660.5	$	1,616.8	$	(4,934.5	)	$	1,155.0
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax	(46.7		)	53.2			53.2		41.0		(106.4		)	(5.7		)
Realized (gains) losses on derivatives recognized in net income, net of tax	1.9			45.5			29.6		19.1		(59.2		)	36.9
Change in pension and postretirement benefit plan liability, net of tax	5.3			(0.7		)	—		0.2		—			4.8
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax	—			3.1			3.1		2.3		(6.1		)	2.4
Total other comprehensive income (loss), net of tax	(39.5		)	101.1			85.9		62.6		(171.7		)	38.4
Comprehensive income	1,112.2			1,761.6			1,746.4		1,679.4		(5,106.2		)	1,193.4
Less: Comprehensive income attributable to noncontrolling interests	—			—			—		3.3		—			3.3
Comprehensive income attributable to ONEOK	$	1,112.2		$	1,761.6		$	1,746.4	$	1,676.1	$	(5,106.2	)	$	1,190.1

	Year Ending December 31, 2017
	Parent Issuer & Guarantor			Subsidiary Issuer & Guarantor			Guarantor Subsidiary			Combined Non-Guarantor Subsidiaries			Consolidating Entries			Total
	(Millions of dollars)
Net income	$	589.2		$	1,215.7		$	1,215.7		$	1,191.5		$	(3,618.6	)	$	593.5
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax	19.1			(72.2		)	(40.6		)	(8.8		)	81.1			(21.4		)
Realized (gains) losses on derivatives recognized in net income, net of tax	2.5			86.5			69.6			44.3			(139.2		)	63.7
Change in pension and postretirement benefit plan liability, net of tax	(4.2		)	—			—			—			—			(4.2		)
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax	—			(1.1		)	(1.1		)	(1.0		)	2.2			(1.0		)
Total other comprehensive income (loss), net of tax	17.4			13.2			27.9			34.5			(55.9		)	37.1
Comprehensive income	606.6			1,228.9			1,243.6			1,226.0			(3,674.5		)	630.6
Less: Comprehensive income attributable to noncontrolling interests	232.4			—			—			4.3			—			236.7
Comprehensive income attributable to ONEOK	$	374.2		$	1,228.9		$	1,243.6		$	1,221.7		$	(3,674.5	)	$	393.9

	Year Ending December 31, 2016
	Parent Issuer & Guarantor			Subsidiary Issuer & Guarantor			Guarantor Subsidiary			Combined Non-Guarantor Subsidiaries			Consolidating Entries			Total
	(Millions of dollars)
Net income	$	738.0		$	1,066.8		$	1,066.8		$	999.4		$	(3,127.5	)	$	743.5
Other comprehensive income (loss), net of tax
Unrealized gains (losses) on derivatives, net of tax	—			(35.8		)	(78.5		)	(108.8		)	192.8			(30.3		)
Realized (gains) losses on derivatives recognized in net income, net of tax	2.1			(10.7		)	(26.4		)	(33.4		)	61.4			(7.0		)
Change in pension and postretirement benefit plan liability, net of tax	(16.7		)	—			—			—			—			(16.7		)
Other comprehensive income (loss) on investments in unconsolidated affiliates, net of tax	—			(1.8		)	(1.8		)	(3.3		)	5.4			(1.5		)
Total other comprehensive income (loss), net of tax	(14.6		)	(48.3		)	(106.7		)	(145.5		)	259.6			(55.5		)
Comprehensive income	723.4			1,018.5			960.1			853.9			(2,867.9		)	688.0
Less: Comprehensive income attributable to noncontrolling interests	357.6			—			—			5.5			—			363.1
Comprehensive income attributable to ONEOK	$	365.8		$	1,018.5		$	960.1		$	848.4		$	(2,867.9	)	$	324.9

Condensed Consolidating Balance Sheets

	December 31, 2018
	Parent Issuer & Guarantor		Subsidiary Issuer & Guarantor		Guarantor Subsidiary		Combined Non-Guarantor Subsidiaries		Consolidating Entries			Total
Assets	(Millions of dollars)
Current assets
Cash and cash equivalents	$	12.0	$	—	$	—	$	—	$	—		$	12.0
Accounts receivable, net	—		—		—		819.0		—			819.0
Materials and supplies	—		—		—		141.2		—			141.2
Natural gas and natural gas liquids in storage	—		—		—		296.7		—			296.7
Other current assets	29.1		—		—		100.6		—			129.7
Total current assets	41.1		—		—		1,357.5		—			1,398.6
Property, plant and equipment
Property, plant and equipment	145.5		—		—		17,885.5		—			18,031.0
Accumulated depreciation and amortization	92.0		—		—		3,172.3		—			3,264.3
Net property, plant and equipment	53.5		—		—		14,713.2		—			14,766.7
Investments and other assets
Investments	6,153.5		3,548.1		9,721.6		791.1		(19,245.1		)	969.2
Intercompany notes receivable	5,308.6		7,701.5		1,528.0		—		(14,538.1		)	—
Other assets	115.9		—		—		982.3		(1.0		)	1,097.2
Total investments and other assets	11,578.0		11,249.6		11,249.6		1,773.4		(33,784.2		)	2,066.4
Total assets	$	11,672.6	$	11,249.6	$	11,249.6	$	17,844.1	$	(33,784.2	)	$	18,231.7
Liabilities and equity
Current liabilities
Current maturities of long-term debt	$	—	$	500.0	$	—	$	7.7	$	—		$	507.7
Accounts payable	31.3		—		—		1,086.8		—			1,118.1
Other current liabilities	123.2		81.0		—		278.4		—			482.6
Total current liabilities	154.5		581.0		—		1,372.9		—			2,108.4
Intercompany debt	—		—		7,701.5		6,836.6		(14,538.1		)	—
Long-term debt, excluding current maturities	4,510.7		4,341.4		—		21.2		—			8,873.3
Deferred credits and other liabilities
Deferred income taxes	112.3		—		—		108.4		(1.0		)	219.7
Other deferred credits	315.6		—		—		135.2		—			450.8
Total deferred credits and other liabilities	427.9		—		—		243.6		(1.0		)	670.5
Commitments and contingencies
Equity	6,579.5		6,327.2		3,548.1		9,369.8		(19,245.1		)	6,579.5
Total liabilities and equity	$	11,672.6	$	11,249.6	$	11,249.6	$	17,844.1	$	(33,784.2	)	$	18,231.7

	December 31, 2017
	Parent Issuer & Guarantor		Subsidiary Issuer & Guarantor		Guarantor Subsidiary		Combined Non-Guarantor Subsidiaries		Consolidating Entries			Total
Assets	(Millions of dollars)
Current assets
Cash and cash equivalents	$	37.2	$	—	$	—	$	—	$	—		$	37.2
Accounts receivable, net	—		—		—		1,203.0		—			1,203.0
Materials and supplies	—		—		—		90.3		—			90.3
Natural gas and natural gas liquids in storage	—		—		—		342.3		—			342.3
Other current assets	9.8		1.3		—		80.6		—			91.7
Total current assets	47.0		1.3		—		1,716.2		—			1,764.5
Property, plant and equipment
Property, plant and equipment	128.3		—		—		15,431.3		—			15,559.6
Accumulated depreciation and amortization	86.4		—		—		2,775.1		—			2,861.5
Net property, plant and equipment	41.9		—		—		12,656.2		—			12,698.1
Investments and other assets
Investments	5,752.1		3,133.7		8,058.4		803.0		(16,744.0		)	1,003.2
Intercompany notes receivable	2,926.9		8,627.8		3,703.1		—		(15,257.8		)	—
Other assets	416.9		0.2		—		1,007.4		(44.4		)	1,380.1
Total investments and other assets	9,095.9		11,761.7		11,761.5		1,810.4		(32,046.2		)	2,383.3
Total assets	$	9,184.8	$	11,763.0	$	11,761.5	$	16,182.8	$	(32,046.2	)	$	16,845.9
Liabilities and equity
Current liabilities
Current maturities of long-term debt	$	—	$	425.0	$	—	$	7.7	$	—		$	432.7
Short-term borrowings	614.7		—		—		—		—			614.7
Accounts payable	12.0		—		—		1,128.6		—			1,140.6
Other current liabilities	65.9		85.0		—		328.4		—			479.3
Total current liabilities	692.6		510.0		—		1,464.7		—			2,667.3
Intercompany debt	—		—		8,627.8		6,630.0		(15,257.8		)	—
Long-term debt, excluding current maturities	2,726.4		5,336.4		—		28.8		—			8,091.6
Deferred credits and other liabilities
Deferred income taxes	—		—		—		97.1		(44.4		)	52.7
Other deferred credits	237.9		—		—		111.0		—			348.9
Total deferred credits and other liabilities	237.9		—		—		208.1		(44.4		)	401.6
Commitments and contingencies
Equity
Equity excluding noncontrolling interests in consolidated subsidiaries	5,527.9		5,916.6		3,133.7		7,693.7		(16,744.0		)	5,527.9
Noncontrolling interests in consolidated subsidiaries	—		—		—		157.5		—			157.5
Total equity	5,527.9		5,916.6		3,133.7		7,851.2		(16,744.0		)	5,685.4
Total liabilities and equity	$	9,184.8	$	11,763.0	$	11,761.5	$	16,182.8	$	(32,046.2	)	$	16,845.9

Condensed Consolidating Statements of Cash Flows

	Year Ended December 31, 2018
	Parent Issuer & Guarantor			Subsidiary Issuer & Guarantor			Guarantor Subsidiary			Combined Non-Guarantor Subsidiaries			Consolidating Entries			Total
	(Millions of dollars)
Operating activities
Cash provided by operating activities	$	1,325.1		$	1,344.7		$	67.9		$	2,113.0		$	(2,664.0	)	$	2,186.7
Investing activities
Capital expenditures	(18.8		)	—			—			(2,122.7		)	—			(2,141.5		)
Other investing activities	—			—			15.3			11.3			—			26.6
Cash used in investing activities	(18.8		)	—			15.3			(2,111.4		)	—			(2,114.9		)
Financing activities
Dividends paid	(1,335.1		)	(1,332.0		)	(1,332.0		)	—			2,664.0			(1,335.1		)
Distributions to noncontrolling interests	—			—			—			(3.5		)	—			(3.5		)
Intercompany borrowings (advances), net	(2,154.4		)	912.3			1,248.8			(6.7		)	—			—
Repayment of short-term borrowings, net	(614.7		)	—			—			—			—			(614.7		)
Issuance of long-term debt, net of discounts	1,795.8			—			—			—			—			1,795.8
Repayment of long-term debt	—			(925.0		)	—			(7.7		)	—			(932.7		)
Issuance of common stock	1,204.0			—			—			—			—			1,204.0
Acquisition of noncontrolling interests	(195.0		)	—			—			—			—			(195.0		)
Other, net	(32.1		)	—			—			16.3			—			(15.8		)
Cash used in financing activities	(1,331.5		)	(1,344.7		)	(83.2		)	(1.6		)	2,664.0			(97.0		)
Change in cash and cash equivalents	(25.2		)	—			—			—			—			(25.2		)
Cash and cash equivalents at beginning of period	37.2			—			—			—			—			37.2
Cash and cash equivalents at end of period	$	12.0		$	—		$	—		$	—		$	—		$	12.0

	Year Ending December 31, 2017
	Parent Issuer & Guarantor			Subsidiary Issuer & Guarantor			Guarantor Subsidiary			Combined Non-Guarantor Subsidiaries			Consolidating Entries			Total
	(Millions of dollars)
Operating activities
Cash provided by operating activities	$	947.4		$	1,348.3		$	59.0		$	1,353.7		$	(2,393.0	)	$	1,315.4
Investing activities
Capital expenditures	—			—			—			(512.4		)	—			(512.4		)
Contributions to unconsolidated affiliates	—			—			(83.0		)	(4.9		)	—			(87.9		)
Other investing activities	—			—			14.8			17.9			—			32.7
Cash used in investing activities	—			—			(68.2		)	(499.4		)	—			(567.6		)
Financing activities
Dividends paid	(829.4		)	(1,332.0		)	(1,332.0		)	—			2,664.0			(829.4		)
Distributions to noncontrolling interests	—			—			—			(5.3		)	(271.0		)	(276.3		)
Intercompany borrowings (advances), net	(2,500.7		)	2,001.2			1,340.8			(841.3		)	—			—
Borrowing (repayment) of short-term borrowings, net	614.7			(1,110.3		)	—			—			—			(495.6		)
Issuance of long-term debt, net of discounts	1,190.5			—			—			—			—			1,190.5
Repayment of long-term debt	(87.1		)	(900.0		)	—			(7.7		)	—			(994.8		)
Issuance of common stock	471.4			—			—			—			—			471.4
Other, net	(18.1		)	(7.2		)	—			—			—			(25.3		)
Cash provided by (used in) financing activities	(1,158.7		)	(1,348.3		)	8.8			(854.3		)	2,393.0			(959.5		)
Change in cash and cash equivalents	(211.3		)	—			(0.4		)	—			—			(211.7		)
Cash and cash equivalents at beginning of period	248.5			—			0.4			—			—			248.9
Cash and cash equivalents at end of period	$	37.2		$	—		$	—		$	—		$	—		$	37.2

	Year Ending December 31, 2016
	Parent Issuer & Guarantor			Subsidiary Issuer & Guarantor			Guarantor Subsidiary			Combined Non-Guarantor Subsidiaries			Consolidating Entries			Total
	(Millions of dollars)
Operating activities
Cash provided by operating activities	$	717.0		$	1,334.5		$	70.0		$	1,353.9		$	(2,122.1	)	$	1,353.3
Investing activities
Capital expenditures	(0.2		)	—			—			(624.4		)	—			(624.6		)
Other investing activities	—			—			34.9			(25.7		)	—			9.2
Cash provided by (used in) investing activities	(0.2		)	—			34.9			(650.1		)	—			(615.4		)
Financing activities
Dividends paid	(517.6		)	(1,332.0		)	(1,332.0		)	—			2,664.0			(517.6		)
Distributions to noncontrolling interests	—			—			—			(7.5		)	(541.9		)	(549.4		)
Intercompany borrowings (advances), net	(63.1		)	(470.8		)	1,222.4			(688.5		)	—			—
Borrowing of short-term borrowings, net	—			563.9			—			—			—			563.9
Issuance of long-term debt, net of discounts	—			1,000.0			—			—			—			1,000.0
Debt financing costs	—			(2.8		)	—			—			—			(2.8		)
Repayment of long-term debt	(0.3		)	(1,100.0		)	—			(7.7		)	—			(1,108.0		)
Issuance of common stock	22.0			—			—			—			—			22.0
Other, net	(1.7		)	7.2			—			(0.1		)	—			5.4
Cash used in financing activities	(560.7		)	(1,334.5		)	(109.6		)	(703.8		)	2,122.1			(586.5		)
Change in cash and cash equivalents	156.1			—			(4.7		)	—			—			151.4
Change in cash and cash equivalents included in discontinued operations	(0.1		)	—			—			—			—			(0.1		)
Change in cash and cash equivalents included in continuing operations	156.0			—			(4.7		)	—			—			151.3
Cash and cash equivalents at beginning of period	92.5			—			5.1			—			—			97.6
Cash and cash equivalents at end of period	$	248.5		$	—		$	0.4		$	—		$	—		$	248.9