EQUITY INVESTMENTS (Details) - USD ($) $ in Millions				1 Months Ended		3 Months Ended				6 Months Ended			12 Months Ended
		Aug. 01, 2018	Jul. 25, 2018	Jun. 30, 2017	Apr. 30, 2006	Jun. 30, 2018		Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017	Dec. 31, 2017	Jul. 31, 2018	Jun. 01, 2017
EQUITY INVESTMENTS
Equity Earnings						$ 36.0			$ 24.0	$ 95.0		$ 60.0
Equity Investments						1,211.0				1,211.0			$ 1,213.0
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 7)						36.0				36.0			$ 51.0
Distributions from Equity Investments
Distributions received from equity investments						56.0			40.0	101.0		68.0
Distributions from equity investments										96.0		68.0
Return on investment distribution classified as investing activities						2.6			0.0	5.0		0.0
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				$ 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				$ 145.0			4.0
Distributions from Equity Investments
Distributions from equity investments		$ 14.0				14.0				28.0
Return on investment distribution classified as investing activities		$ 2.6				$ 2.6				$ 5.2
Limited partners, Distribution declared			$ 29.0
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)						50.00%	[1]			50.00%	[1]			50.00%
Equity Earnings	[1]					$ 15.0			15.0	$ 32.0		34.0
Equity Investments	[1]					505.0				505.0			512.0
Amortization period of transaction fee					12 years
Transaction fee					$ 10.0
Additional ownership interest acquired (as a percent)					20.00%
Assets
Cash and cash equivalents						11.0				11.0			14.0
Other current assets						34.0				34.0			36.0
Plant, property and equipment, net						1,056.0				1,056.0			1,063.0
Other assets						14.0				14.0			14.0
Assets, total						1,115.0				1,115.0			1,127.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities						37.0				37.0			38.0
Deferred credits and other						33.0				33.0			31.0
Net long-term debt, including current maturities						264.0				264.0			264.0
Partners' capital						782.0				782.0			795.0
Accumulated other comprehensive loss						(1.0)				(1.0)			(1.0)
Liabilities and Partners' Equity, total						1,115.0				1,115.0			1,127.0
Revenues (expenses)
Transmission revenues						68.0			69.0	140.0		144.0
Operating expenses						(19.0)			(18.0)	(38.0)		(36.0)
Depreciation						(15.0)			(15.0)	(30.0)		(30.0)
Financial charges and other						(4.0)			(5.0)	(7.0)		(9.0)
Net income						$ 30.0			31.0	$ 65.0		69.0
Great Lakes
EQUITY INVESTMENTS
Ownership interest (as a percent)		46.45%				46.45%		46.45%		46.45%
Equity Earnings						$ 12.0			6.0	$ 36.0		23.0
Equity Investments						485.0				485.0			479.0
Equity contribution								$ 4.0		4.0		4.0
Assets
Current assets						89.0				89.0			107.0
Plant, property and equipment, net						695.0				695.0			701.0
Assets, total						784.0				784.0			808.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities						49.0				49.0			75.0
Net long-term debt, including current maturities						250.0				250.0			259.0
Current portion of long-term debt (Note 7)						0.0				0.0			0.0
Other non-current liabilities													1.0
Partners' capital						485.0				485.0			473.0
Liabilities and Partners' Equity, total						784.0				784.0			808.0
Revenues (expenses)
Transmission revenues						53.0			41.0	134.0		103.0
Operating expenses						(15.0)			(17.0)	(32.0)		(30.0)
Depreciation						(8.0)			(7.0)	(16.0)		(14.0)
Financial charges and other						(5.0)			(5.0)	(9.0)		(10.0)
Net income						$ 25.0			12.0	$ 77.0		49.0
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)						49.34%				49.34%					49.34%
Equity Earnings						$ 9.0			3.0	$ 27.0		3.0
Equity Investments						221.0				221.0			222.0
Undistributed earnings						0.0			$ 0.0	0.0		$ 0.0
Assets
Cash and cash equivalents						100.0				100.0			86.0
Other current assets						26.0				26.0			36.0
Plant, property and equipment, net						584.0				584.0			591.0
Other assets						10.0				10.0			8.0
Assets, total						720.0				720.0			721.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities						17.0				17.0			17.0
Net long-term debt, including current maturities						327.0				327.0			329.0
Other non-current liabilities						13.0				13.0			9.0
Partners' capital						363.0				363.0			366.0
Liabilities and Partners' Equity, total						720.0				720.0			$ 721.0
Revenues (expenses)
Transmission revenues				$ 14.0		44.0				105.0
Operating expenses				(5.0)		(14.0)				(28.0)
Depreciation				(2.0)		(7.0)				(15.0)
Financial charges and other				(1.0)		(4.0)				(7.0)
Net income				$ 6.0		$ 19.0				$ 55.0

[1] OwnershipEquity EarningsEquity InvestmentsInterest atThree monthsSix Months(unaudited)June 30, ended June 30, ended June 30, June 30, December 31, (millions of dollars)2018    20182017    2018201720182017Northern Border (a) 50% 15  15 32  34 505 512Great Lakes 46.45% 12 6 36 23 485 479Iroquois(b) 49.34% 9  3 27  3 221 222  36  24 95  60 1,211 1,213Equity earnings from Northern Border is net of the 12-year amortization of a $10 million transaction fee paid to the operator of Northern Border at the time of the Partnership’s acquisition of an additional 20 percent interest in April 2006.Equity earnings from Iroquois is net of the 29-year amortization of a $10 million purchase price discrepancy assumed by the Partnership from TransCanada at the time of the 2017 Acquisition.