EQUITY INVESTMENTS (Details) - USD ($) $ in Millions			1 Months Ended		3 Months Ended			9 Months Ended
	Dec. 30, 2019	Nov. 01, 2019	Aug. 31, 2019	Jun. 30, 2019	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
EQUITY INVESTMENTS
Equity Earnings					$ 31.0		$ 34.0	$ 115.0	$ 129.0
Equity Investments					1,094.0			1,094.0		$ 1,196.0
Distributions from Equity Investments
Distributions received from equity investments					59.0		49.0	226.0	150.0
Distributions from equity investments								168.0	142.0
Return on investment distribution classified as investing activities					2.6			57.8
Northern Border
EQUITY INVESTMENTS
Special distribution from equity investments								100.0
Borrowings under the revolving credit facility				$ 100.0
Distribution financed by borrowings (as a percent)				100.00%
Northern Border | Revolving credit facility
EQUITY INVESTMENTS
Maximum borrowing capacity				$ 200.0
Great Lakes
EQUITY INVESTMENTS
Total cash call issued to fund debt repayment								11.0
Iroquois
EQUITY INVESTMENTS
Total cash call issued to fund debt repayment			$ 7.0
Distributions from Equity Investments
Return on investment distribution classified as investing activities		$ 2.6			$ 2.6		2.6	$ 7.8	7.8
Limited partners, Distribution declared		$ 28.0
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)					50.00%			50.00%
Equity Earnings					$ 15.0		16.0	$ 50.0	49.0
Equity Investments					426.0			426.0		497.0
ASSETS
Cash and cash equivalents					38.0			38.0		10.0
Other current assets					34.0			34.0		36.0
Property, plant and equipment, net					1,000.0			1,000.0		1,037.0
Other assets					13.0			13.0		13.0
Assets, total					1,085.0			1,085.0		1,096.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					60.0			60.0		34.0
Deferred credits and other					37.0			37.0		35.0
Net long-term debt, including current maturities					365.0			365.0		264.0
Partners' capital					624.0			624.0		764.0
Accumulated other comprehensive loss					(1.0)			(1.0)		(1.0)
Liabilities and Partners' Equity, total					1,085.0			1,085.0		1,096.0
Revenues (expenses)
Transmission revenues					73.0		72.0	221.0	212.0
Operating expenses					(21.0)		(19.0)	(61.0)	(57.0)
Depreciation					(15.0)		(15.0)	(46.0)	(45.0)
Financial charges and other					(5.0)		(5.0)	(13.0)	(12.0)
Net income					32.0		33.0	101.0	98.0
Long-term debt current maturities					0.0			0.0		0.0
Distributions from Equity Investments
Distributions received from equity investments					$ 21.0		21.0	121.0	60.0
Return on investment distribution classified as investing activities								$ 50.0
Great Lakes
EQUITY INVESTMENTS
Ownership interest (as a percent)					46.45%			46.45%
Equity Earnings					$ 8.0		9.0	$ 37.0	45.0
Equity Investments					482.0			482.0		489.0
Equity contribution						$ 5.0		5.0	4.0
ASSETS
Current assets					59.0			59.0		75.0
Property, plant and equipment, net					685.0			685.0		689.0
Assets, total					744.0			744.0		764.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					29.0			29.0		26.0
Net long-term debt, including current maturities					229.0			229.0		240.0
Other non-current liabilities					5.0			5.0		4.0
Partners' capital					481.0			481.0		494.0
Liabilities and Partners' Equity, total					744.0			744.0		764.0
Revenues (expenses)
Transmission revenues					51.0		49.0	174.0	183.0
Operating expenses					(23.0)		(17.0)	(58.0)	(50.0)
Depreciation					(8.0)		(8.0)	(24.0)	(24.0)
Financial charges and other					(3.0)		(5.0)	(12.0)	(13.0)
Net income					17.0		19.0	80.0	96.0
Long-term debt current maturities					$ 21.0			$ 21.0		21.0
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)	49.34%	49.34%	49.34%		49.34%			49.34%
Equity Earnings					$ 8.0		9.0	$ 28.0	35.0
Equity Investments					186.0			186.0		210.0
Equity contribution			$ 4.0					4.0
ASSETS
Cash and cash equivalents					38.0			38.0		80.0
Other current assets					33.0			33.0		32.0
Property, plant and equipment, net					570.0			570.0		581.0
Other assets					14.0			14.0		8.0
Assets, total					655.0			655.0		701.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					21.0			21.0		19.0
Net long-term debt, including current maturities					320.0			320.0		325.0
Other non-current liabilities					20.0			20.0		14.0
Partners' capital					294.0			294.0		343.0
Liabilities and Partners' Equity, total					655.0			655.0		701.0
Revenues (expenses)
Transmission revenues					39.0		42.0	131.0	147.0
Operating expenses					(15.0)		(13.0)	(43.0)	(41.0)
Depreciation					(7.0)		(7.0)	(22.0)	(22.0)
Financial charges and other					(2.0)		(4.0)	(9.0)	(11.0)
Net income					15.0		18.0	57.0	73.0
Long-term debt current maturities					5.0			5.0		$ 146.0
Distributions from Equity Investments
Distributions from equity investments	$ 14.0				$ 28.0		$ 14.0	56.0	42.0
Return on investment distribution classified as investing activities								$ 8.0	$ 8.0