Amount of distribution to be paid to its non-controlling interest owners.

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Aggregate amount of cash distribution paid to general partner of limited partnership (LP). Includes ordinary distributions and incentive distributions paid pursuant to the partnership agreement.

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Represents the combined amount of cash distributions paid to the general partner interest and unit-holder of limited partnership (LP), excluding any distribution made to limited partner based on distributable cash flow of subsidiary.

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percentage of reduction in cash distributions.

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Amount of equity impact of cash distribution declared to unit-holder of limited partnership (LP).

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Amount of cash distribution paid to unit-holder of limited partnership (LP).

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Per unit of ownership amount of cash distributions declared to unit-holder of a limited partnership (LP).

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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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Total distributions to each class of partners (i.e., general, limited and preferred partners).

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SUBSEQUENT EVENTS - Distributions (Details) - USD ($) $ / shares in Units, $ in Millions							1 Months Ended	3 Months Ended			9 Months Ended
	Nov. 02, 2018	Nov. 01, 2018	Oct. 23, 2018	Oct. 22, 2018	Oct. 17, 2018	Oct. 10, 2018	Oct. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018
Distributions
Partnership distribution											$ 197
Partnership's share of distributions											$ 142	$ 106
Northern Border
Distributions
Ownership interest (as a percent)								50.00%			50.00%
Great Lakes
Distributions
Ownership interest (as a percent)		46.45%						46.45%			46.45%		46.45%
Subsequent Events
Distributions
Total cash distribution	$ 47
Percentage of reduction in cash distributions	35.00%
Subsequent Events \| Northern Border
Distributions
Ownership interest (as a percent)							50.00%
Subsequent Events \| Distribution declared \| Portland Natural Gas Transmission System
Distributions
Partnership distribution			$ 8
Subsequent Events \| Distribution declared \| Northern Border
Distributions
Partnership distribution						$ 15
Subsequent Events \| Distribution declared \| Great Lakes
Distributions
Partnership distribution					$ 22
Subsequent Events \| Cash Distribution Paid \| Northern Border
Distributions
Partnership's share of distributions							$ 7
Subsequent Events \| Cash Distribution Paid \| Great Lakes
Distributions
Partnership's share of distributions		$ 10
Subsequent Events \| Cash Distribution Paid \| Portland Natural Gas Transmission System
Distributions
Share of distributions to its non-controlling interest owner		$ 3
General Partner
Distributions
Partnership distribution											$ 7
TC PipeLines GP, Inc. \| Subsequent Events
Distributions
General Partner cash distributions	$ 1
Ownership interest (as a percent)	2.00%
Common Units
Distributions
Per Unit Distribution, declared (in dollars per unit)									$ 1.00
Total cash distribution								$ 47		$ 74	$ 171	$ 210
Number of units								71,300,000		69,600,000	71,300,000	69,600,000
Common Units \| Subsequent Events
Distributions
Per Unit Distribution, declared (in dollars per unit)				$ 0.65
Limited partners, Distribution declared	$ 46
Common Units \| Limited Partners
Distributions
Partnership distribution											$ 164
Common Units \| TC PipeLines GP, Inc. \| Subsequent Events
Distributions
Limited Partners, Distributions paid	$ 4
Common Units \| TC PipeLines GP, Inc. \| Limited Partners
Distributions
Number of units	5,797,106
Common Units \| TransCanada \| Subsequent Events
Distributions
Limited Partners, Distributions paid	$ 7
Common Units \| TransCanada \| TC PipeLines GP, Inc.
Distributions
Number of units	11,287,725