EQUITY INVESTMENTS - Narrative (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule of Equity Method Investments [Line Items]
Distributions received from equity investments	$ 70,000,000	$ 59,000,000	$ 190,000,000	$ 226,000,000
Return on investment distribution classified as investing activities			29,000,000	58,000,000
Partnership's share of distributions			161,000,000	168,000,000
Northern Border
Schedule of Equity Method Investments [Line Items]
Return on investment distribution classified as investing activities			0	50,000,000
Partnership's share of distributions	$ 22,000,000	21,000,000	$ 68,000,000	121,000,000
Ownership interest	50.00%		50.00%
Great Lakes
Schedule of Equity Method Investments [Line Items]
Partnership's share of distributions	$ 11,000,000	$ 9,000,000	$ 48,000,000	48,000,000
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]
Distributions received from equity investments	$ 37,000,000	$ 28,000,000	$ 74,000,000	$ 56,000,000
Return on investment distribution classified as investing activities			29,000,000	8,000,000
Equity contribution			$ 0	$ 4,000,000
Ownership interest	49.34%	49.34%	49.34%	49.34%
Great Lakes
Schedule of Equity Method Investments [Line Items]
Total cash call issued to fund debt repayment			$ 11,000,000
Iroquois
Schedule of Equity Method Investments [Line Items]
Return on investment distribution classified as investing activities	$ 0	$ 2,600,000	$ 5,200,000	$ 7,800,000
Partnership's share of distributions	2,000,000
Iroquois \| Special distribution, terminated project guaranteed proceeds
Schedule of Equity Method Investments [Line Items]
Return on investment distribution classified as investing activities	$ 24,000,000

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Distributions received from equity investments relating to both operating and investing activities.

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Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, classified as investing activities.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

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Amount of distribution received from equity method investee for return of investment, classified as investing activities. Excludes distribution for return on investment, classified as operating activities.

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