EQUITY INVESTMENTS (Details) - USD ($) $ in Millions				1 Months Ended	3 Months Ended
		May 01, 2018	Mar. 07, 2018	Apr. 30, 2006	Mar. 31, 2018		Mar. 31, 2017		Apr. 30, 2018	Dec. 31, 2017	Jun. 01, 2017
EQUITY INVESTMENTS
Equity Earnings					$ 59.0		$ 36.0	[1]
Equity Investments					1,217.0					$ 1,213.0
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 7)					45.0					51.0
Distributions from Equity Investments
Distributions received from equity investments					45.0		28.0
Distributions from equity investments					43.0		28.0	[1]
Return on investment distribution classified as investing activities					2.0		0.0
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
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)		49.34%			49.34%						49.34%
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 7)					$ 4.0					4.0
Revenues (expenses)
Transmission revenues					60.0
Operating expenses					(14.0)
Depreciation					(7.0)
Financial charges and other					(4.0)
Net income					35.0
Distributions from Equity Investments
Distributions from equity investments		$ 14.0			14.0
Return on investment distribution classified as investing activities		$ 2.6			$ 2.0
Limited partners, Distribution declared			$ 29.0
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)					50.00%	[2]			50.00%
Equity Earnings	[2]				$ 17.0		19.0
Equity Investments	[2]				507.0					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					22.0					14.0
Other current assets					35.0					36.0
Plant, property and equipment, net					1,059.0					1,063.0
Other assets					14.0					14.0
Assets, total					1,130.0					1,127.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					50.0					38.0
Deferred credits and other					32.0					31.0
Net long-term debt, including current maturities					264.0					264.0
Partners' equity
Partners' capital					785.0					795.0
Accumulated other comprehensive loss					(1.0)					(1.0)
Liabilities and Partners' Equity, total					1,130.0					1,127.0
Revenues (expenses)
Transmission revenues					72.0		74.0
Operating expenses					(19.0)		(17.0)
Depreciation					(15.0)		(15.0)
Financial charges and other					(4.0)		(4.0)
Net income					$ 34.0		38.0
Great Lakes
EQUITY INVESTMENTS
Ownership interest (as a percent)		46.45%			46.45%
Equity Earnings					$ 24.0		17.0
Equity Investments					499.0					479.0
Equity contribution					4.0		4.0	[1]
Assets
Current assets					129.0					107.0
Plant, property and equipment, net					699.0					701.0
Assets, total					828.0					808.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					63.0					75.0
Net long-term debt, including current maturities					250.0					259.0
Current portion of long-term debt (Note 7)					0.0					0.0
Other non-current liabilities										1.0
Partners' equity
Partners' capital					515.0					473.0
Liabilities and Partners' Equity, total					828.0					808.0
Revenues (expenses)
Transmission revenues					81.0		63.0
Operating expenses					(17.0)		(14.0)
Depreciation					(8.0)		(7.0)
Financial charges and other					(4.0)		(5.0)
Net income					$ 52.0		$ 37.0
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)					49.34%	[3]					49.34%
Equity Earnings	[3]				$ 18.0
Equity Investments	[3]				211.0					222.0
Undistributed earnings					0.0
Assets
Cash and cash equivalents					105.0					86.0
Other current assets					32.0					36.0
Plant, property and equipment, net					589.0					591.0
Other assets					9.0					8.0
Assets, total					735.0					721.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					50.0					17.0
Net long-term debt, including current maturities					329.0					329.0
Other non-current liabilities					12.0					9.0
Partners' equity
Partners' capital					344.0					366.0
Liabilities and Partners' Equity, total					$ 735.0					$ 721.0

[1] Recast to consolidate PNGTS (Refer to Note 2).

[2] OwnershipEquity EarningsEquity InvestmentsInterest atThree months(unaudited)March 31, ended March 31, March 31, December 31, (millions of dollars)2018    2018201720182017Northern Border (a) 50% 17  19 507 512Great Lakes 46.45% 24 17 499 479Iroquois(b) 49.34% 18  — 211 222  59  36 1,217 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.

[3] The Partnership acquired a 49.34% interest in Iroquois on June 1, 2017.