EQUITY INVESTMENTS (Details) - USD ($) $ in Millions					3 Months Ended			6 Months Ended
	Aug. 31, 2019	Aug. 01, 2019	Jul. 31, 2019	Jun. 26, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 26, 2029	Dec. 31, 2018
EQUITY INVESTMENTS
Equity Earnings					$ 30.0		$ 36.0	$ 84.0	$ 95.0
Equity Investments					1,118.0			1,118.0			$ 1,196.0
Equity contribution								5.0	4.0
Distributions from Equity Investments
Distributions received from equity investments					108.0		56.0	167.0	101.0
Distributions from equity investments								112.0	96.0
Return on investment distribution classified as investing activities					52.6			55.2
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
Distributions from Equity Investments
Distributions from equity investments		$ 14.0			14.0		14.0	28.0	28.0
Return on investment distribution classified as investing activities	$ 2.6				$ 2.6		2.6	$ 5.2	5.2
Limited partners, Distribution declared			$ 28.0
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)					50.00%			50.00%
Equity Earnings					$ 14.0		15.0	$ 35.0	32.0
Equity Investments					431.0			431.0			497.0
ASSETS
Cash and cash equivalents					26.0			26.0			10.0
Other current assets					33.0			33.0			36.0
Property, plant and equipment, net					1,008.0			1,008.0			1,037.0
Other assets					13.0			13.0			13.0
Assets, total					1,080.0			1,080.0			1,096.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					46.0			46.0			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					633.0			633.0			764.0
Accumulated other comprehensive loss					(1.0)			(1.0)			(1.0)
Liabilities and Partners' Equity, total					1,080.0			1,080.0			1,096.0
Revenues (expenses)
Transmission revenues					67.0		68.0	148.0	140.0
Operating expenses					(20.0)		(19.0)	(40.0)	(38.0)
Depreciation					(16.0)		(15.0)	(31.0)	(30.0)
Financial charges and other					(4.0)		(4.0)	(8.0)	(7.0)
Net income					27.0		30.0	69.0	65.0
Long-term debt current maturities					0.0			0.0			0.0
Distributions from Equity Investments
Distributions received from equity investments					$ 72.0		17.0	100.0	39.0
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					$ 9.0		12.0	$ 29.0	36.0
Equity Investments					484.0			484.0			489.0
Equity contribution						$ 5.0			4.0
ASSETS
Current assets					56.0			56.0			75.0
Property, plant and equipment, net					687.0			687.0			689.0
Assets, total					743.0			743.0			764.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					26.0			26.0			26.0
Net long-term debt, including current maturities					229.0			229.0			240.0
Other non-current liabilities					4.0			4.0			4.0
Partners' capital					484.0			484.0			494.0
Liabilities and Partners' Equity, total					743.0			743.0			764.0
Revenues (expenses)
Transmission revenues					51.0		53.0	123.0	134.0
Operating expenses					(19.0)		(15.0)	(35.0)	(32.0)
Depreciation					(8.0)		(8.0)	(16.0)	(16.0)
Financial charges and other					(5.0)		(5.0)	(9.0)	(9.0)
Net income					19.0		25.0	63.0	77.0
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%
Equity Earnings					$ 7.0		9.0	$ 20.0	27.0
Equity Investments					203.0			203.0			210.0
ASSETS
Cash and cash equivalents					73.0			73.0			80.0
Other current assets					25.0			25.0			32.0
Property, plant and equipment, net					573.0			573.0			581.0
Other assets					14.0			14.0			8.0
Assets, total					685.0			685.0			701.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					15.0			15.0			19.0
Net long-term debt, including current maturities					319.0			319.0			325.0
Other non-current liabilities					21.0			21.0			14.0
Partners' capital					330.0			330.0			343.0
Liabilities and Partners' Equity, total					685.0			685.0			701.0
Revenues (expenses)
Transmission revenues					40.0		44.0	92.0	105.0
Operating expenses					(13.0)		(14.0)	(28.0)	(28.0)
Depreciation					(8.0)		(7.0)	(15.0)	(15.0)
Financial charges and other					(4.0)		(4.0)	(7.0)	(7.0)
Net income					15.0		$ 19.0	42.0	55.0
Long-term debt current maturities					$ 4.0			4.0			$ 146.0
Distributions from Equity Investments
Return on investment distribution classified as investing activities								$ 5.0	$ 5.0