EQUITY INVESTMENTS - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Equity Method Investments [Line Items]
Distributions received from equity investments	$ 49,000,000	$ 108,000,000	$ 120,000,000	$ 167,000,000
Return on investment distribution classified as investing activities			5,000,000	55,000,000
Partnership's share of distributions			115,000,000	112,000,000
Iroquois
Schedule of Equity Method Investments [Line Items]
Return on investment distribution classified as investing activities			$ 5,000,000	$ 5,000,000
Ownership interest	49.34%	49.34%	49.34%	49.34%
Partnership's share of distributions	$ 10,000,000	$ 14,000,000	$ 38,000,000	$ 28,000,000
Northern Border
Schedule of Equity Method Investments [Line Items]
Distributions received from equity investments	$ 18,000,000	72,000,000	45,000,000	100,000,000
Return on investment distribution classified as investing activities			$ 0	50,000,000
Ownership interest	50.00%		50.00%
Great Lakes
Schedule of Equity Method Investments [Line Items]
Distributions received from equity investments	$ 21,000,000	$ 23,000,000	$ 37,000,000	39,000,000
Equity contribution			$ 5,000,000	$ 5,000,000
Ownership interest	46.45%	46.45%	46.45%	46.45%
Iroquois
Schedule of Equity Method Investments [Line Items]
Return on investment distribution classified as investing activities	$ 0	$ 2,600,000	$ 5,200,000	$ 5,200,000
Great Lakes
Schedule of Equity Method Investments [Line Items]
Total cash call issued to fund debt repayment			$ 11,000,000