EQUITY INVESTMENTS - Summary of equity investments (Details) - USD ($)	1 Months Ended	3 Months Ended								12 Months Ended
	Apr. 30, 2006	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 06, 2020	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Equity Earnings		$ 47,000,000	$ 39,000,000	$ 29,000,000	$ 55,000,000	$ 45,000,000	$ 31,000,000	$ 30,000,000	$ 54,000,000	$ 170,000,000	$ 160,000,000	$ 173,000,000
Equity Investments		1,070,000,000				1,098,000,000				1,070,000,000	1,098,000,000
ASSETS
Current assets		257,000,000				156,000,000				257,000,000	156,000,000
Property, plant and equipment, net		1,747,000,000				1,528,000,000				1,747,000,000	1,528,000,000
Cash and cash equivalents		200,000,000				83,000,000				200,000,000	83,000,000
Other current assets		6,000,000				6,000,000				6,000,000	6,000,000
TOTAL ASSETS		3,145,000,000				2,853,000,000				3,145,000,000	2,853,000,000
LIABILITIES AND PARTNERS' EQUITY
Current liabilities		487,000,000				170,000,000				487,000,000	170,000,000
Long-term debt, net		2,198,000,000								2,198,000,000
Deferred credits and other		47,000,000				36,000,000				47,000,000	36,000,000
Partners’ equity		833,000,000				760,000,000				833,000,000	760,000,000	699,000,000		$ 1,068,000,000
Accumulated other comprehensive loss		(13,000,000)				(5,000,000)				(13,000,000)	(5,000,000)
TOTAL LIABILITIES AND PARTNERS' EQUITY		3,145,000,000				2,853,000,000				3,145,000,000	2,853,000,000
Revenues (expenses)
Transmission revenues		104,000,000	$ 99,000,000	$ 95,000,000	$ 101,000,000	104,000,000	$ 93,000,000	$ 93,000,000	$ 113,000,000	399,000,000	403,000,000	549,000,000
Depreciation										(89,000,000)	(78,000,000)	(97,000,000)
Financial charges and other										(73,000,000)	(83,000,000)	(92,000,000)
Net income (loss)										284,000,000	280,000,000	(182,000,000)
Current maturities		423,000,000				123,000,000				423,000,000	123,000,000
Northern Border
ASSETS
Property, plant and equipment, net		977,000,000				989,000,000				977,000,000	989,000,000
Cash and cash equivalents		31,000,000				21,000,000				31,000,000	21,000,000
Other current assets		38,000,000				37,000,000				38,000,000	37,000,000
Other assets		12,000,000				12,000,000				12,000,000	12,000,000
TOTAL ASSETS		1,058,000,000				1,059,000,000				1,058,000,000	1,059,000,000
LIABILITIES AND PARTNERS' EQUITY
Current liabilities		52,000,000				42,000,000				52,000,000	42,000,000
Long-term debt, net		380,000,000				364,000,000				380,000,000	364,000,000
Deferred credits and other		42,000,000				39,000,000				42,000,000	39,000,000
Partners’ capital		584,000,000				615,000,000				584,000,000	615,000,000
Accumulated other comprehensive loss		0				(1,000,000)				0	(1,000,000)
TOTAL LIABILITIES AND PARTNERS' EQUITY		1,058,000,000				1,059,000,000				1,058,000,000	1,059,000,000
Revenues (expenses)
Transmission revenues										308,000,000	300,000,000	289,000,000
Operating expenses										(77,000,000)	(82,000,000)	(78,000,000)
Depreciation										(62,000,000)	(62,000,000)	(60,000,000)
Financial charges and other										(18,000,000)	(18,000,000)	(15,000,000)
Net income (loss)										151,000,000	138,000,000	136,000,000
Current maturities		$ 250,000,000				0				$ 250,000,000	0
Stated interest rate		7.50%								7.50%
Great Lakes
ASSETS
Current assets		$ 66,000,000				72,000,000				$ 66,000,000	72,000,000
Property, plant and equipment, net		716,000,000				685,000,000				716,000,000	685,000,000
TOTAL ASSETS		782,000,000				757,000,000				782,000,000	757,000,000
LIABILITIES AND PARTNERS' EQUITY
Current liabilities		38,000,000				33,000,000				38,000,000	33,000,000
Long-term debt, net		198,000,000				219,000,000				198,000,000	219,000,000
Deferred credits and other		9,000,000				6,000,000				9,000,000	6,000,000
Partners’ equity		537,000,000				499,000,000				537,000,000	499,000,000
TOTAL LIABILITIES AND PARTNERS' EQUITY		782,000,000				757,000,000				782,000,000	757,000,000
Revenues (expenses)
Transmission revenues										239,000,000	238,000,000	246,000,000
Operating expenses										(70,000,000)	(79,000,000)	(68,000,000)
Depreciation										(33,000,000)	(32,000,000)	(32,000,000)
Financial charges and other										(15,000,000)	(16,000,000)	(18,000,000)
Net income (loss)										121,000,000	111,000,000	128,000,000
Current maturities		31,000,000				21,000,000				31,000,000	21,000,000
Iroquois
ASSETS
Property, plant and equipment, net		506,000,000				570,000,000				506,000,000	570,000,000
Cash and cash equivalents		25,000,000				43,000,000				25,000,000	43,000,000
Other current assets		36,000,000				36,000,000				36,000,000	36,000,000
Other assets		20,000,000				16,000,000				20,000,000	16,000,000
TOTAL ASSETS		587,000,000				665,000,000				587,000,000	665,000,000
LIABILITIES AND PARTNERS' EQUITY
Current liabilities		20,000,000				34,000,000				20,000,000	34,000,000
Long-term debt, net		314,000,000				317,000,000				314,000,000	317,000,000
Deferred credits and other		21,000,000				20,000,000				21,000,000	20,000,000
Partners’ equity		232,000,000				294,000,000				232,000,000	294,000,000
TOTAL LIABILITIES AND PARTNERS' EQUITY		587,000,000				665,000,000				587,000,000	665,000,000
Revenues (expenses)
Transmission revenues										183,000,000	180,000,000	194,000,000
Operating expenses										(59,000,000)	(58,000,000)	(57,000,000)
Depreciation										(30,000,000)	(29,000,000)	(29,000,000)
Financial charges and other										(15,000,000)	(11,000,000)	(14,000,000)
Net income (loss)										79,000,000	82,000,000	94,000,000
Current maturities		$ 5,000,000				3,000,000				$ 5,000,000	3,000,000
Northern Border
Schedule of Equity Method Investments [Line Items]
Interest acquired		50.00%								50.00%
Equity Earnings										$ 76,000,000	69,000,000	68,000,000
Equity Investments		$ 407,000,000				422,000,000				407,000,000	422,000,000
Transaction fee amortization period	12 years
Transaction fee	$ 10,000,000
Additional ownership percentage	20.00%
Equity Method Investment, Other than Temporary Impairment										$ 0	0	0
Great Lakes
Schedule of Equity Method Investments [Line Items]
Interest acquired		46.45%								46.45%
Equity Earnings										$ 56,000,000	51,000,000	59,000,000
Equity Investments		$ 509,000,000				491,000,000				$ 509,000,000	491,000,000
Iroquois
Schedule of Equity Method Investments [Line Items]
Interest acquired		49.34%								49.34%			49.34%
Equity Earnings										$ 38,000,000	40,000,000	$ 46,000,000
Equity Investments		$ 154,000,000				$ 185,000,000				$ 154,000,000	$ 185,000,000