EQUITY INVESTMENTS - Summary of equity investments (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Equity Earnings	$ 39,000,000	$ 31,000,000	$ 123,000,000	$ 115,000,000
Equity Investments	1,051,000,000		1,051,000,000		$ 1,098,000,000
ASSETS
Cash and cash equivalents	253,000,000		253,000,000		83,000,000
Other current assets	3,000,000		3,000,000		6,000,000
Current assets	300,000,000		300,000,000		156,000,000
Property, plant and equipment, net	1,702,000,000		1,702,000,000		1,528,000,000
TOTAL ASSETS	3,124,000,000		3,124,000,000		2,853,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	499,000,000		499,000,000		170,000,000
Other liabilities	48,000,000		48,000,000		36,000,000
Long-term debt, net	2,147,000,000		2,147,000,000
Accumulated other comprehensive income (loss) (AOCI)	(16,000,000)		(16,000,000)		(5,000,000)
Partners’ equity	804,000,000		804,000,000		760,000,000
TOTAL LIABILITIES AND PARTNERS’ EQUITY	3,124,000,000		3,124,000,000		2,853,000,000
NET INCOME
Transmission revenues	99,000,000	93,000,000	295,000,000	299,000,000
Depreciation and amortization	(29,000,000)	(19,000,000)	(68,000,000)	(58,000,000)
Financial charges and other	(17,000,000)	(20,000,000)	(54,000,000)	(63,000,000)
Current portion of long-term debt	373,000,000		373,000,000		123,000,000
Northern Border
ASSETS
Cash and cash equivalents	40,000,000		40,000,000		21,000,000
Other current assets	39,000,000		39,000,000		37,000,000
Property, plant and equipment, net	979,000,000		979,000,000		989,000,000
Other assets	12,000,000		12,000,000		12,000,000
TOTAL ASSETS	1,070,000,000		1,070,000,000		1,059,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	58,000,000		58,000,000		42,000,000
Other liabilities	40,000,000		40,000,000		39,000,000
Long-term debt, net	378,000,000		378,000,000		364,000,000
Partners’ capital	594,000,000		594,000,000		615,000,000
Accumulated other comprehensive income (loss) (AOCI)	0		0		(1,000,000)
TOTAL LIABILITIES AND PARTNERS’ EQUITY	1,070,000,000		1,070,000,000		1,059,000,000
NET INCOME
Transmission revenues	83,000,000	73,000,000	232,000,000	221,000,000
Operating expenses	(19,000,000)	(21,000,000)	(58,000,000)	(61,000,000)
Depreciation and amortization	(16,000,000)	(15,000,000)	(47,000,000)	(46,000,000)
Financial charges and other	(4,000,000)	(5,000,000)	(13,000,000)	(13,000,000)
Net income	44,000,000	32,000,000	114,000,000	101,000,000
Current portion of long-term debt	$ 250,000,000		$ 250,000,000		0
Stated interest rate	7.50%		7.50%
Great Lakes
ASSETS
Current assets	$ 27,000,000		$ 27,000,000		72,000,000
Property, plant and equipment, net	710,000,000		710,000,000		685,000,000
TOTAL ASSETS	737,000,000		737,000,000		757,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	31,000,000		31,000,000		33,000,000
Other liabilities	8,000,000		8,000,000		6,000,000
Long-term debt, net	208,000,000		208,000,000		219,000,000
Partners’ equity	490,000,000		490,000,000		499,000,000
TOTAL LIABILITIES AND PARTNERS’ EQUITY	737,000,000		737,000,000		757,000,000
NET INCOME
Transmission revenues	50,000,000	51,000,000	172,000,000	174,000,000
Operating expenses	(18,000,000)	(23,000,000)	(53,000,000)	(58,000,000)
Depreciation and amortization	(8,000,000)	(8,000,000)	(24,000,000)	(24,000,000)
Financial charges and other	(4,000,000)	(3,000,000)	(11,000,000)	(12,000,000)
Net income	20,000,000	17,000,000	84,000,000	80,000,000
Current portion of long-term debt	31,000,000		31,000,000		21,000,000
Iroquois
ASSETS
Cash and cash equivalents	26,000,000		26,000,000		43,000,000
Other current assets	32,000,000		32,000,000		36,000,000
Property, plant and equipment, net	509,000,000		509,000,000		570,000,000
Other assets	19,000,000		19,000,000		16,000,000
TOTAL ASSETS	586,000,000		586,000,000		665,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	21,000,000		21,000,000		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	227,000,000		227,000,000		294,000,000
TOTAL LIABILITIES AND PARTNERS’ EQUITY	586,000,000		586,000,000		665,000,000
NET INCOME
Transmission revenues	40,000,000	39,000,000	133,000,000	131,000,000
Operating expenses	(15,000,000)	(15,000,000)	(44,000,000)	(43,000,000)
Depreciation and amortization	(7,000,000)	(7,000,000)	(22,000,000)	(22,000,000)
Financial charges and other	(3,000,000)	(2,000,000)	(12,000,000)	(9,000,000)
Net income	15,000,000	15,000,000	55,000,000	57,000,000
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	$ 22,000,000	$ 15,000,000	$ 57,000,000	$ 50,000,000
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	$ 10,000,000	$ 8,000,000	$ 39,000,000	$ 37,000,000
Equity Investments	$ 487,000,000		$ 487,000,000		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	$ 8,000,000	$ 27,000,000	$ 28,000,000
Equity Investments	$ 152,000,000		$ 152,000,000		$ 185,000,000