RELATED PARTY TRANSACTIONS (Details) $ in Millions			1 Months Ended	12 Months Ended
	Oct. 31, 2019 USD ($)	Nov. 01, 2017	Sep. 21, 2017 Bcf	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Nov. 01, 2020 USD ($)
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				$ 8.0	$ 6.0
Amount included in receivables from related party					2.0
General Partner | Reimbursement of costs of services provided
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				4.0	4.0	$ 4.0
Great Lakes | TransCanada's subsidiaries
Capital and operating costs charged to the pipeline systems and amount payable
Amount included in receivables from related party				19.0	18.0
Great Lakes | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				5.0	3.0
Termination options beginning		3 years
Great Lakes | TC Energy | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				47.0	44.0	36.0
Impact on the Partnership's net income attributable to controlling interests				20.0	19.0	15.0
Amount included in receivables from related party				34.0	36.0
Great Lakes | TC Energy | Transportation contracts
Capital and operating costs charged to the pipeline systems and amount payable
Contract term		10 years
Total revenue earned				$ 76.0	$ 76.0	$ 13.0
Great Lakes | TC Energy | Transportation contracts | Total net revenues | Customer concentration risk
Capital and operating costs charged to the pipeline systems and amount payable
Percent of total revenues				73.00%	73.00%	57.00%
Great Lakes | TC Energy | Affiliated rental revenue
Capital and operating costs charged to the pipeline systems and amount payable
Percent of total revenues						1.00%
Great Lakes | TC Energy | Affiliated rental revenue | Maximum
Capital and operating costs charged to the pipeline systems and amount payable
Percent of total revenues				1.00%	1.00%
Great Lakes | ANR Pipeline Company | Transportation contracts
Capital and operating costs charged to the pipeline systems and amount payable
Contract term					15 years
Total contract value					$ 1,300.0
Transportation capacity per day | Bcf			0.711
Northern Border | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				$ 4.0	3.0
Northern Border | TC Energy | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				39.0	36.0	$ 43.0
Impact on the Partnership's net income attributable to controlling interests				18.0	16.0	16.0
PNGTS | TransCanada's subsidiaries | Transportation contracts
Capital and operating costs charged to the pipeline systems and amount payable
Amount included in receivables from related party				0.0	0.0
Revenues from related party				1.0	1.0	1.0
PNGTS | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				1.0	1.0
PNGTS | TC Energy | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				7.0	9.0	9.0
Impact on the Partnership's net income attributable to controlling interests				4.0	5.0	5.0
PNGTS | TC Energy | Portland XPress expansion project (PXP), Phase III | PXP Phase III
Capital and operating costs charged to the pipeline systems and amount payable
Costs reimbursable at reporting date				$ 0.6
PNGTS | TC Energy | Portland XPress expansion project (PXP), Phase III | PXP Phase III | Forecast
Capital and operating costs charged to the pipeline systems and amount payable
Costs reimbursable at reporting date							$ 8.0
PNGTS | Affiliates | Construction of facilities | Minimum
Capital and operating costs charged to the pipeline systems and amount payable
Reimbursement of costs	$ 140.0
PNGTS | Affiliates | Portland XPress expansion project (PXP), Phase III
Capital and operating costs charged to the pipeline systems and amount payable
Expansion Project, Phase In Period				3 years
GTN | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				$ 5.0	4.0
GTN | TC Energy | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				45.0	34.0	34.0
Impact on the Partnership's net income attributable to controlling interests				$ 33.0	$ 28.0	$ 29.0
Percentage of capital and operating costs charged				100.00%	100.00%	100.00%
Bison | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable					$ 1.0
Bison | TC Energy | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				$ 2.0	6.0	$ 6.0
Impact on the Partnership's net income attributable to controlling interests				2.0	6.0	6.0
North Baja | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				1.0
North Baja | TC Energy | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				5.0	4.0	4.0
Impact on the Partnership's net income attributable to controlling interests				4.0	4.0	4.0
Tuscarora | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable					1.0
Tuscarora | TC Energy | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				4.0	4.0	4.0
Impact on the Partnership's net income attributable to controlling interests				$ 4.0	$ 4.0	$ 4.0