RELATED PARTY TRANSACTIONS (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended
	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2017
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	$ 6.0						$ 5.0
Amount included in receivables from related party	3.0						1.0
Great Lakes
Capital and operating costs charged to the pipeline systems and amount payable
Estimated revenue sharing provision							40.0
General Partner | Reimbursement of costs of services provided
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	1.0		$ 1.0
TransCanada's subsidiaries | Great Lakes
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	3.0						3.0
Percentage of capital and operating costs charged		100.00%		100.00%	100.00%	100.00%
Amount included in receivables from related party	10.0						20.0
TransCanada's subsidiaries | Great Lakes | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	$ 9.0		$ 8.0
Percentage of capital and operating costs charged	100.00%		100.00%
TransCanada's subsidiaries | Northern Border
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	$ 4.0						4.0
Percentage of capital and operating costs charged		100.00%		100.00%	100.00%	100.00%
TransCanada's subsidiaries | Northern Border | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	9.0		$ 10.0
Percentage of capital and operating costs charged		100.00%		100.00%	100.00%	100.00%
TransCanada's subsidiaries | Portland Natural Gas Transmission System
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	1.0						1.0
Amount included in receivables from related party	0.0						0.0
TransCanada's subsidiaries | Portland Natural Gas Transmission System | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	2.0		2.0
TransCanada's subsidiaries | Iroquois
Capital and operating costs charged to the pipeline systems and amount payable
Percentage of capital and operating costs charged		100.00%			100.00%
TransCanada's subsidiaries | Iroquois | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Percentage of capital and operating costs charged		100.00%			100.00%
TransCanada's subsidiaries | GTN
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	3.0						3.0
TransCanada's subsidiaries | GTN | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	8.0		7.0
Impact on the Partnership's net income attributable to controlling interests	8.0		7.0
TransCanada's subsidiaries | Bison
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	1.0						1.0
TransCanada's subsidiaries | Bison | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	2.0		1.0
Impact on the Partnership's net income attributable to controlling interests	2.0		1.0
TransCanada's subsidiaries | North Baja Pipeline, LLC | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	1.0		1.0
Impact on the Partnership's net income attributable to controlling interests	1.0		1.0
TransCanada's subsidiaries | Tuscarora Gas Transmission Company | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	1.0		1.0
Impact on the Partnership's net income attributable to controlling interests	1.0		1.0
TransCanada's subsidiaries | Great Lakes | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Impact on the Partnership's net income attributable to controlling interests	$ 4.0		$ 3.0
TransCanada's subsidiaries | Great Lakes | Transportation contracts | Total net revenues | Customer concentration risk
Capital and operating costs charged to the pipeline systems and amount payable
Percent of total revenues	68.00%		67.00%
TransCanada's subsidiaries | Northern Border | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Impact on the Partnership's net income attributable to controlling interests	$ 4.0		$ 3.0
TransCanada's subsidiaries | Portland Natural Gas Transmission System | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Impact on the Partnership's net income attributable to controlling interests	1.0		$ 1.0
TransCanada's subsidiaries | Portland Natural Gas Transmission System | Transportation contracts
Capital and operating costs charged to the pipeline systems and amount payable
Revenues from related party	1.0						$ 0.0
Affiliates | Portland Natural Gas Transmission System
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	$ 5.0