RELATED PARTY TRANSACTIONS (Details) - USD ($) $ in Millions			3 Months Ended			4 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	May. 03, 2015	Dec. 31, 2014	Jan. 01, 2016	Apr. 01, 2015
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	$ 4.0	$ 5.0	$ 4.0	$ 5.0
Great Lakes
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)	46.45%		46.45%
Northern Border
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)	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%
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	$ 2.0	3.0	2.0	3.0
Net amounts (receivable)	$ (15.0)	$ (17.0)	(15.0)	(17.0)
Percentage of capital and operating costs charged	100.00%	100.00%
Amount included in receivables from related party	$ 15.0	$ 17.0	15.0	17.0
TransCanada's subsidiaries | Great Lakes | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged			$ 7.0		$ 6.0
Percentage of capital and operating costs charged			100.00%		100.00%
TransCanada's subsidiaries | Great Lakes | Transportation contracts
Capital and operating costs charged to the pipeline systems and amount payable
Revenues from related party			$ 46.0		$ 29.0
TransCanada's subsidiaries | Great Lakes | Transportation contracts | Total revenues | Customer concentration risk
Capital and operating costs charged to the pipeline systems and amount payable
Percent of total revenues			76.00%		61.00%
TransCanada's subsidiaries | Northern Border
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	$ 2.0	$ 5.0	$ 2.0	5.0
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			$ 6.0		$ 7.0
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
Percentage of capital and operating costs charged	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
Percentage of capital and operating costs charged			100.00%
TransCanada's subsidiaries | GTN
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	$ 2.0	$ 3.0	$ 2.0	3.0
TransCanada's subsidiaries | GTN | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged			6.0		$ 6.0
Impact on the Partnership's net income attributable to controlling interests			$ 5.0		$ 4.0
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 (receivable)	(1.0)		$ (1.0)
Amount included in receivables from related party	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					$ 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 | 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	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 charged to the pipeline systems and amount payable
Revenue from TransCanada and its affiliates included in the Partnership's equity earnings			22.0		14.0
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 | 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
Former parent, TransCanada subsidiaries | Transaction between entities under common control | GTN
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)									30.00%
ANR Pipeline Company | Great Lakes | Firm service between Michigan and Wisconsin
Capital and operating costs charged to the pipeline systems and amount payable
Deferred revenue related to services performed						$ 14.0	$ 9.0
Deferred revenue recognized				$ 23.0