EQUITY INVESTMENTS (Details) - USD ($) $ in Millions			1 Months Ended	3 Months Ended			4 Months Ended	9 Months Ended		12 Months Ended
	Nov. 01, 2018	Oct. 22, 2018	Apr. 30, 2006	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jun. 01, 2017
EQUITY INVESTMENTS
Equity Earnings				$ 34.0		$ 27.0		$ 129.0	$ 87.0
Equity Investments				1,196.0			$ 1,213.0	1,196.0		$ 1,213.0
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 7)				36.0			51.0	36.0		$ 51.0
Distributions from Equity Investments
Distributions received from equity investments								150.0	109.0
Distributions from equity investments								142.0	106.0
Return on investment distribution classified as investing activities								8.0	3.0
TransCanada
EQUITY INVESTMENTS
Amortization period of purchase price discrepancy										29 years
Purchase price discrepancy										$ 10.0
Great Lakes
EQUITY INVESTMENTS
Total cash call issued to fund debt repayment					$ 9.0
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 7)				$ 21.0			19.0	$ 21.0		19.0
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)	49.34%			49.34%				49.34%			49.34%
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 7)				$ 145.0			4.0	$ 145.0		4.0
Distributions from Equity Investments
Distributions from equity investments	$ 14.0							42.0
Return on investment distribution classified as investing activities	$ 2.6							$ 7.8
Limited partners, Distribution declared		$ 29.0
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)				50.00%				50.00%
Equity Earnings				$ 16.0		16.0		$ 49.0	50.0
Equity Investments				501.0			512.0	501.0		512.0
Amortization period of transaction fee			12 years
Transaction fee			$ 10.0
Additional ownership interest acquired (as a percent)			20.00%
Equity contribution									83.0
Assets
Cash and cash equivalents				17.0			14.0	17.0		14.0
Other current assets				34.0			36.0	34.0		36.0
Property, plant and equipment, net				1,048.0			1,063.0	1,048.0		1,063.0
Other assets				14.0			14.0	14.0		14.0
Assets, total				1,113.0			1,127.0	1,113.0		1,127.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities				43.0			38.0	43.0		38.0
Deferred credits and other				34.0			31.0	34.0		31.0
Net long-term debt, including current maturities				264.0			264.0	264.0		264.0
Partners' capital				773.0			795.0	773.0		795.0
Accumulated other comprehensive loss				(1.0)			(1.0)	(1.0)		(1.0)
Liabilities and Partners' Equity, total				1,113.0			1,127.0	1,113.0		1,127.0
Revenues (expenses)
Transmission revenues				72.0		73.0		212.0	217.0
Operating expenses				(19.0)		(20.0)		(57.0)	(56.0)
Depreciation				(15.0)		(15.0)		(45.0)	(45.0)
Financial charges and other				(5.0)		(5.0)		(12.0)	(14.0)
Net income				$ 33.0		33.0		$ 98.0	102.0
Great Lakes
EQUITY INVESTMENTS
Ownership interest (as a percent)	46.45%			46.45%	46.45%			46.45%
Equity Earnings				$ 9.0		2.0		$ 45.0	24.0
Equity Investments				480.0			479.0	480.0		479.0
Equity contribution					$ 4.0			4.0	4.0
Assets
Current assets				53.0			107.0	53.0		107.0
Property, plant and equipment, net				693.0			701.0	693.0		701.0
Assets, total				746.0			808.0	746.0		808.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities				18.0			75.0	18.0		75.0
Net long-term debt, including current maturities				250.0			259.0	250.0		259.0
Current portion of long-term debt (Note 7)				0.0			0.0	0.0		0.0
Other non-current liabilities				3.0			1.0	3.0		1.0
Partners' capital				475.0			473.0	475.0		473.0
Liabilities and Partners' Equity, total				746.0			808.0	746.0		808.0
Revenues (expenses)
Transmission revenues				49.0		34.0		183.0	138.0
Operating expenses				(17.0)		(19.0)		(50.0)	(49.0)
Depreciation				(8.0)		(7.0)		(24.0)	(21.0)
Financial charges and other				(5.0)		(5.0)		(13.0)	(16.0)
Net income				$ 19.0		3.0		$ 96.0	52.0
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)				49.34%				49.34%			49.34%
Equity Earnings				$ 9.0		9.0		$ 35.0	13.0
Equity Investments				215.0			222.0	215.0		222.0
Undistributed earnings				0.0		0.0		0.0	$ 0.0
Assets
Cash and cash equivalents				91.0			86.0	91.0		86.0
Other current assets				34.0			36.0	34.0		36.0
Property, plant and equipment, net				582.0			591.0	582.0		591.0
Other assets				9.0			8.0	9.0		8.0
Assets, total				716.0			721.0	716.0		721.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities				23.0			17.0	23.0		17.0
Net long-term debt, including current maturities				327.0			329.0	327.0		329.0
Other non-current liabilities				13.0			9.0	13.0		9.0
Partners' capital				353.0			366.0	353.0		366.0
Liabilities and Partners' Equity, total				716.0			721.0	716.0		$ 721.0
Revenues (expenses)
Transmission revenues				42.0		43.0	57.0	147.0
Operating expenses				(13.0)		(13.0)	(18.0)	(41.0)
Depreciation				(7.0)		(7.0)	(9.0)	(22.0)
Financial charges and other				(4.0)		(4.0)	(5.0)	(11.0)
Net income				$ 18.0		$ 19.0	$ 25.0	$ 73.0