EQUITY INVESTMENTS - Summary of equity investments (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Equity Earnings	$ 55,000,000	$ 54,000,000
Equity Investments	$ 1,102,000,000		$ 1,098,000,000
Northern Border
Schedule of Equity Method Investments [Line Items]
Ownership Interest	50.00%
Equity Earnings	$ 22,000,000	21,000,000
Equity Investments	416,000,000		422,000,000
ASSETS
Cash and cash equivalents	25,000,000		21,000,000
Other current assets	38,000,000		37,000,000
Property, plant and equipment, net	979,000,000		989,000,000
Other assets	12,000,000		12,000,000
Assets, total	1,054,000,000		1,059,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	47,000,000		42,000,000
Deferred credits and other	40,000,000		39,000,000
Net long-term debt, including current maturities	364,000,000		364,000,000
Partners’ capital	604,000,000		615,000,000
Accumulated other comprehensive loss	(1,000,000)		(1,000,000)
Liabilities and Partners' Equity, total	1,054,000,000		1,059,000,000
NET INCOME
Transmission revenues	83,000,000	81,000,000
Operating expenses	(20,000,000)	(20,000,000)
Depreciation	(15,000,000)	(15,000,000)
Financial charges and other	(4,000,000)	(4,000,000)
Net income	44,000,000	42,000,000
Long-term debt current maturities	$ 0		0
Great Lakes
Schedule of Equity Method Investments [Line Items]
Ownership Interest	46.45%
Equity Earnings	$ 20,000,000	20,000,000
Equity Investments	501,000,000		491,000,000
ASSETS
Current assets	78,000,000		72,000,000
Property, plant and equipment, net	684,000,000		685,000,000
Assets, total	762,000,000		757,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	28,000,000		33,000,000
Net long-term debt, including current maturities	208,000,000		219,000,000
Other long term liabilities	7,000,000		6,000,000
Partners’ capital	519,000,000		499,000,000
Liabilities and Partners' Equity, total	762,000,000		757,000,000
NET INCOME
Transmission revenues	72,000,000	72,000,000
Operating expenses	(16,000,000)	(16,000,000)
Depreciation	(8,000,000)	(8,000,000)
Financial charges and other	(4,000,000)	(4,000,000)
Net income	44,000,000	44,000,000
Long-term debt current maturities	$ 31,000,000		21,000,000
Iroquois
Schedule of Equity Method Investments [Line Items]
Ownership Interest	49.34%
Equity Earnings	$ 13,000,000	13,000,000
Equity Investments	185,000,000		185,000,000
ASSETS
Cash and cash equivalents	32,000,000		43,000,000
Other current assets	86,000,000		36,000,000
Property, plant and equipment, net	515,000,000		570,000,000
Other assets	17,000,000		16,000,000
Assets, total	650,000,000		665,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	21,000,000		34,000,000
Net long-term debt, including current maturities	317,000,000		317,000,000
Other long term liabilities	19,000,000		20,000,000
Partners’ capital	293,000,000		294,000,000
Liabilities and Partners' Equity, total	650,000,000		665,000,000
NET INCOME
Transmission revenues	52,000,000	52,000,000
Operating expenses	(15,000,000)	(15,000,000)
Depreciation	(8,000,000)	(7,000,000)
Financial charges and other	(3,000,000)	(3,000,000)
Net income	26,000,000	$ 27,000,000
Long-term debt current maturities	$ 3,000,000		$ 3,000,000