RELATED PARTY TRANSACTIONS (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Nov. 01, 2020	Dec. 31, 2018
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	$ 6.0			$ 6.0			$ 6.0
Amount included in receivables from related party	0.0			0.0			2.0
General Partner | Reimbursement of costs of services provided
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged		$ 1.0			$ 3.0
ANR Pipeline Company | Great Lakes | Transportation contracts
Capital and operating costs charged to the pipeline systems and amount payable
Contract term			15 years
Total contract value			$ 1,300.0
Great Lakes | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	5.0			5.0			3.0
Amount included in receivables from related party	13.0			13.0			18.0
Great Lakes | TC Energy | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	12.0	9.0		35.0	34.0
Impact on the Partnership's net income attributable to controlling interests	$ 4.0	$ 4.0		$ 14.0	$ 14.0
Percentage of capital and operating costs charged				100.00%	100.00%
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%	76.00%		73.00%	71.00%
Northern Border | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	$ 4.0			$ 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	10.0	$ 8.0		29.0	$ 26.0
Impact on the Partnership's net income attributable to controlling interests	4.0	4.0		$ 13.0	$ 12.0
Percentage of capital and operating costs charged				100.00%	100.00%
PNGTS | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	1.0			$ 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	2.0	2.0		5.0	$ 7.0
Impact on the Partnership's net income attributable to controlling interests	1.0	1.0		$ 3.0	$ 4.0
Percentage of capital and operating costs charged				100.00%	100.00%
PNGTS | TC Energy | Portland XPress expansion project (PXP), Phase III
Capital and operating costs charged to the pipeline systems and amount payable
Costs Incurred, Development Costs, Reimbursable To Related Party	0.0			$ 0.0
PNGTS | TC Energy | Portland XPress expansion project (PXP), Phase III | Forecast
Capital and operating costs charged to the pipeline systems and amount payable
Costs Incurred, Development Costs, Reimbursable To Related Party						$ 7.2
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	4.0			$ 4.0			4.0
GTN | TC Energy | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	11.0	8.0		32.0	$ 25.0
Impact on the Partnership's net income attributable to controlling interests	9.0	7.0		25.0	21.0
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	1.0	2.0		2.0	5.0
Impact on the Partnership's net income attributable to controlling interests		2.0		1.0	5.0
North Baja | TC Energy
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	1.0			1.0
North Baja | TC Energy | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	1.0	1.0		4.0	3.0
Impact on the Partnership's net income attributable to controlling interests	1.0	1.0		3.0	3.0
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	1.0	1.0		3.0	3.0
Impact on the Partnership's net income attributable to controlling interests	1.0	$ 1.0		3.0	$ 3.0
Affiliates | PXP
Capital and operating costs charged to the pipeline systems and amount payable
Costs Incurred, Development Costs	134.0			134.0
Affiliates | PXP Phase III
Capital and operating costs charged to the pipeline systems and amount payable
Costs Incurred, Development Costs	$ 0.0			$ 0.0