ACQUISITIONS - 2017 Acquisition and 2016 PNGTS Acquisition (Details)			1 Months Ended	12 Months Ended
	Aug. 01, 2017 USD ($) item	Jun. 01, 2017 USD ($)	Aug. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Feb. 20, 2020 USD ($)	Jan. 06, 2020
PNGTS
ACQUISITIONS
Ownership interest, including acquired interest (as a percent)		61.71%
Outstanding debt		$ 5,000,000
PNGTS purchase price
Current assets		25,000,000
Property, plant and equipment, net		294,000,000
Current liabilities		(4,000,000)
Deferred state income taxes		(10,000,000)
Long-term debt, including current portion		(41,000,000)
Net assets		264,000,000
Non-controlling interest		(100,000,000)
Carrying value of pre-existing Investment in PNGTS		(132,000,000)
TransCanada's carrying value of the acquired 11.81 percent interest at June 1, 2017		32,000,000
Excess purchase price over net assets acquired		21,000,000
Total cash consideration		53,000,000
Purchase price before assumption of debt		55,000,000
Final working capital adjustment		3,000,000
Reduction in partner's equity due to excess purchase price		$ 21,000,000
Iroquois
ACQUISITIONS
Interest acquired (as a percent)	49.34%	49.34%		49.34%		49.34%
Option to acquire (as a percent)		0.66
Amount of final purchase price adjustments		$ 19,000,000
Additional consideration on surplus cash		28,000,000
Purchase price before assumption of debt		710,000,000
Outstanding debt		164,000,000
Payment for option to acquire		1,000
Net purchase price
Net Purchase Price		593,000,000
Less: TransCanada's carrying value of Iroquois at June 1, 2017		223,000,000
Excess purchase price		370,000,000
Net purchase price		710,000,000	$ 4,000,000	$ 4,000,000
Reduction in partner's equity under equity method investments		370,000,000
Iroquois | PNGTS
ACQUISITIONS
Purchase price		765,000,000
Amount of final purchase price adjustments		$ 50,000,000
Iroquois | Investing Activities
ACQUISITIONS
Expected return on investment distribution classified as investing activities	$ 28,400,000
Number of quarters for distribution of surplus cash | item	11
Iroquois | TC Energy
ACQUISITIONS
Expected return on investment distribution classified as investing activities	$ 28,000,000
Iroquois | Cash Distribution Paid
ACQUISITIONS
Distributions received to date					$ 25,800,000
PNGTS
ACQUISITIONS
Interest acquired (as a percent)		11.81%
Interest acquired by Partnership (as a percent)		11.81%
Purchase price		$ 55,000,000
Amount of final purchase price adjustments		3,000,000
Outstanding debt		$ 5,000,000