RELATED PARTY TRANSACTIONS (Details) - USD ($) $ in Millions			3 Months Ended		12 Months Ended
	Apr. 18, 2017	Mar. 31, 2017	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	May 01, 2017	Apr. 28, 2017
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable		$ 6.0	$ 6.0		$ 7.0
Amount included in receivables from related party		$ 1.0	1.0		2.0
Partnership distribution			90.0
Partnership's share of distributions			$ 31.0	$ 41.0
Great Lakes
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)		46.45%	46.45%			46.45%
Great Lakes | FERC
Capital and operating costs charged to the pipeline systems and amount payable
Estimated revenue sharing provision			$ 3.4		7.2
Northern Border
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)		50.00%	50.00%				50.00%
Portland Natural Gas Transmission System
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)	49.90%	49.90%	49.90%
Portland Natural Gas Transmission System | Distribution declared
Capital and operating costs charged to the pipeline systems and amount payable
Partnership distribution		$ 5.0
Portland Natural Gas Transmission System | Cash Distribution Paid
Capital and operating costs charged to the pipeline systems and amount payable
Partnership's share of distributions	$ 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	1.0
TransCanada's subsidiaries | GTN
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable		3.0	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			7.0	6.0
Impact on the Partnership's net income attributable to controlling interests			$ 7.0	$ 5.0
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		3.0	$ 3.0		4.0
Percentage of capital and operating costs charged			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			$ 10.0	$ 6.0
Percentage of capital and operating costs charged			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		1.0
Percentage of capital and operating costs charged			100.00%	100.00%
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
Percentage of capital and operating costs charged			100.00%	100.00%
TransCanada's subsidiaries | Bison
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable					1.0
TransCanada's subsidiaries | Bison | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged			$ 1.0
Revenue, net of costs charged				$ (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 charged to the pipeline systems and amount payable
Net amounts payable		3.0	$ 3.0		4.0
Percentage of capital and operating costs charged			100.00%	100.00%
Amount included in receivables from related party		16.0	$ 16.0		19.0
TransCanada's subsidiaries | Great Lakes | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged			$ 8.0	$ 7.0
Percentage of capital and operating costs charged			100.00%	100.00%
TransCanada's subsidiaries | North Baja Pipeline, LLC
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable					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 charged to the pipeline systems and amount payable
Net amounts payable		$ 1.0	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			$ 3.0	$ 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			67.00%	76.00%
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			$ 3.0	$ 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