EQUITY INVESTMENTS - Summary of equity investments (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Equity Earnings	$ 29,000,000	$ 30,000,000	$ 84,000,000	$ 84,000,000
Equity Investments	1,082,000,000		1,082,000,000		$ 1,098,000,000
ASSETS
Cash and cash equivalents	215,000,000		215,000,000		83,000,000
Other current assets	3,000,000		3,000,000		6,000,000
Current assets	262,000,000		262,000,000		156,000,000
Property, plant and equipment, net	1,598,000,000		1,598,000,000		1,528,000,000
TOTAL ASSETS	3,013,000,000		3,013,000,000		2,853,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	415,000,000		415,000,000		170,000,000
Other liabilities	43,000,000		43,000,000		36,000,000
Long-term debt, net	2,119,000,000		2,119,000,000
Accumulated other comprehensive income (loss) (AOCI)	(18,000,000)		(18,000,000)		(5,000,000)
Partners’ equity	787,000,000		787,000,000		760,000,000
TOTAL LIABILITIES AND PARTNERS’ EQUITY	3,013,000,000		3,013,000,000		2,853,000,000
NET INCOME
Transmission revenues	95,000,000	93,000,000	196,000,000	206,000,000
Depreciation and amortization	(19,000,000)	(19,000,000)	(39,000,000)	(39,000,000)
Financial charges and other	(18,000,000)	(21,000,000)	(37,000,000)	(43,000,000)
Net income attributable to controlling interests	57,000,000	55,000,000	145,000,000	148,000,000
Current portion of long-term debt	350,000,000		350,000,000		123,000,000
Northern Border
ASSETS
Cash and cash equivalents	17,000,000		17,000,000		21,000,000
Other current assets	38,000,000		38,000,000		37,000,000
Property, plant and equipment, net	978,000,000		978,000,000		989,000,000
Other assets	12,000,000		12,000,000		12,000,000
TOTAL ASSETS	1,045,000,000		1,045,000,000		1,059,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	42,000,000		42,000,000		42,000,000
Other liabilities	41,000,000		41,000,000		39,000,000
Long-term debt, net	369,000,000		369,000,000		364,000,000
Partners’ capital	594,000,000		594,000,000		615,000,000
Accumulated other comprehensive income (loss) (AOCI)	(1,000,000)		(1,000,000)		(1,000,000)
TOTAL LIABILITIES AND PARTNERS’ EQUITY	1,045,000,000		1,045,000,000		1,059,000,000
NET INCOME
Transmission revenues	66,000,000	67,000,000	149,000,000	148,000,000
Operating expenses	(19,000,000)	(20,000,000)	(39,000,000)	(40,000,000)
Depreciation and amortization	(16,000,000)	(16,000,000)	(31,000,000)	(31,000,000)
Financial charges and other	(5,000,000)	(4,000,000)	(9,000,000)	(8,000,000)
Net income attributable to controlling interests	26,000,000	27,000,000	70,000,000	69,000,000
Current portion of long-term debt	0		0		0
Great Lakes
ASSETS
Current assets	58,000,000		58,000,000		72,000,000
Property, plant and equipment, net	684,000,000		684,000,000		685,000,000
TOTAL ASSETS	742,000,000		742,000,000		757,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	34,000,000		34,000,000		33,000,000
Other liabilities	7,000,000		7,000,000		6,000,000
Long-term debt, net	208,000,000		208,000,000		219,000,000
Partners’ equity	493,000,000		493,000,000		499,000,000
TOTAL LIABILITIES AND PARTNERS’ EQUITY	742,000,000		742,000,000		757,000,000
NET INCOME
Transmission revenues	50,000,000	51,000,000	122,000,000	123,000,000
Operating expenses	(19,000,000)	(19,000,000)	(35,000,000)	(35,000,000)
Depreciation and amortization	(8,000,000)	(8,000,000)	(16,000,000)	(16,000,000)
Financial charges and other	(3,000,000)	(5,000,000)	(7,000,000)	(9,000,000)
Net income attributable to controlling interests	20,000,000	19,000,000	64,000,000	63,000,000
Current portion of long-term debt	31,000,000		31,000,000		21,000,000
Iroquois
ASSETS
Cash and cash equivalents	35,000,000		35,000,000		43,000,000
Other current assets	72,000,000		72,000,000		36,000,000
Property, plant and equipment, net	513,000,000		513,000,000		570,000,000
Other assets	19,000,000		19,000,000		16,000,000
TOTAL ASSETS	639,000,000		639,000,000		665,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	14,000,000		14,000,000		34,000,000
Other liabilities	22,000,000		22,000,000		20,000,000
Long-term debt, net	316,000,000		316,000,000		317,000,000
Partners’ equity	287,000,000		287,000,000		294,000,000
TOTAL LIABILITIES AND PARTNERS’ EQUITY	639,000,000		639,000,000		665,000,000
NET INCOME
Transmission revenues	41,000,000	40,000,000	93,000,000	92,000,000
Operating expenses	(14,000,000)	(13,000,000)	(29,000,000)	(28,000,000)
Depreciation and amortization	(7,000,000)	(8,000,000)	(15,000,000)	(15,000,000)
Financial charges and other	(6,000,000)	(4,000,000)	(9,000,000)	(7,000,000)
Net income attributable to controlling interests	14,000,000	15,000,000	40,000,000	42,000,000
Current portion of long-term debt	$ 4,000,000		$ 4,000,000		3,000,000
Northern Border
Schedule of Equity Method Investments [Line Items]
Ownership Interest	50.00%		50.00%
Equity Earnings	$ 13,000,000	$ 14,000,000	$ 35,000,000	$ 35,000,000
Equity Investments	$ 412,000,000		$ 412,000,000		422,000,000
Great Lakes
Schedule of Equity Method Investments [Line Items]
Ownership Interest	46.45%	46.45%	46.45%	46.45%
Equity Earnings	$ 9,000,000	$ 9,000,000	$ 29,000,000	$ 29,000,000
Equity Investments	$ 489,000,000		$ 489,000,000		491,000,000
Iroquois
Schedule of Equity Method Investments [Line Items]
Ownership Interest	49.34%	49.34%	49.34%	49.34%
Equity Earnings	$ 7,000,000	$ 7,000,000	$ 20,000,000	$ 20,000,000
Equity Investments	$ 181,000,000		$ 181,000,000		$ 185,000,000