RELATED PARTY TRANSACTIONS (Details) - USD ($) $ in Millions			1 Months Ended	3 Months Ended		4 Months Ended	9 Months Ended		12 Months Ended
	Oct. 15, 2015	Dec. 31, 2014	Apr. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	May. 03, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Apr. 01, 2015	Oct. 01, 2014
Capital and operating costs charged to the pipeline systems and amount payable
Accounts payable to affiliates		$ 15.0		$ 8.0			$ 8.0		$ 15.0
Amount included in receivables from related party		1.0		1.0			1.0		1.0
Additional acquisition consideration relating to Carty Lateral project								$ 25.0
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		2.0	2.0
TransCanada's subsidiaries | Great Lakes
Capital and operating costs charged to the pipeline systems and amount payable
Accounts payable to affiliates		9.0		5.0			5.0		9.0
Amount included in receivables from related party		$ 15.0		30.0			30.0		15.0
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	7.0		21.0	22.0
Impact on the Partnership's net income				$ 3.0	$ 3.0		$ 9.0	$ 10.0
Percentage of capital and operating costs charged		100.00%		100.00%	100.00%		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				$ 18.0	$ 13.0		$ 65.0	$ 48.0
TransCanada's subsidiaries | Great Lakes | Transportation contracts | Total revenues | Major customers
Capital and operating costs charged to the pipeline systems and amount payable
Percent of total revenues				62.00%	46.00%		62.00%	46.00%
TransCanada's subsidiaries | Great Lakes | Affiliated rental revenue
Capital and operating costs charged to the pipeline systems and amount payable
Revenues from related party				$ 0.0	$ 0.0		$ 1.0	$ 1.0
TransCanada's subsidiaries | Northern Border
Capital and operating costs charged to the pipeline systems and amount payable
Accounts payable to affiliates		$ 10.0		7.0			7.0		10.0
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	9.0		26.0	26.0
Impact on the Partnership's net income				$ 3.0	$ 4.0		$ 10.0	$ 11.0
Percentage of capital and operating costs charged		100.00%		100.00%	100.00%		100.00%	100.00%
TransCanada's subsidiaries | GTN
Capital and operating costs charged to the pipeline systems and amount payable
Accounts payable to affiliates		$ 10.0		$ 5.0			$ 5.0		10.0
TransCanada's subsidiaries | GTN | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				8.0	$ 8.0		22.0	$ 21.0
Impact on the Partnership's net income				$ 6.0	$ 5.0		$ 17.0	$ 14.0
Percentage of capital and operating costs charged				100.00%	100.00%		100.00%	100.00%
TransCanada's subsidiaries | Bison
Capital and operating costs charged to the pipeline systems and amount payable
Accounts payable to affiliates		2.0		$ 1.0			$ 1.0		2.0
TransCanada's subsidiaries | Bison | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				1.0	$ 2.0		3.0	$ 4.0
Impact on the Partnership's net income				$ 1.0	$ 1.0		$ 3.0	$ 3.0
Percentage of capital and operating costs charged				100.00%	100.00%		100.00%	100.00%
TransCanada's subsidiaries | North Baja Pipeline, LLC
Capital and operating costs charged to the pipeline systems and amount payable
Accounts payable to affiliates		1.0		$ 1.0			$ 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		4.0	$ 3.0
Impact on the Partnership's net income				1.0	1.0		4.0	3.0
TransCanada's subsidiaries | Tuscarora Gas Transmission Company
Capital and operating costs charged to the pipeline systems and amount payable
Accounts payable to affiliates		$ 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		3.0	3.0
Impact on the Partnership's net income				1.0	1.0		3.0	3.0
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				$ 9.0	$ 6.0		$ 31.0	$ 23.0
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
ANR Pipeline Company | Great Lakes | Firm service between Michigan and Wisconsin | Expected
Capital and operating costs charged to the pipeline systems and amount payable
Deferred revenue recognized	$ 23.0
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%
Additional acquisition consideration relating to Carty Lateral project			$ 25.0
Former parent, TransCanada subsidiaries | Transaction between entities under common control | Bison
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)											30.00%