RELATED PARTY TRANSACTIONS (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016		Nov. 01, 2017	Oct. 31, 2017	Sep. 01, 2017	Aug. 01, 2017	Jun. 01, 2017
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	$ 5.0						$ 5.0		$ 8.0	[1]
Amount included in receivables from related party									2.0
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	$ 12.0						$ 22.0		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%	50.00%
Iroquois
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)	49.34%						49.34%							49.34%	49.34%
Portland Natural Gas Transmission System
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)	11.81%		49.90%				11.81%	49.90%							11.81%
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				$ 3.0	$ 3.0
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%	100.00%	100.00%
Amount included in receivables from related party	8.0						8.0		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	$ 10.0		$ 8.0				$ 27.0	$ 22.0
Percentage of capital and operating costs charged	100.00%		100.00%				100.00%	100.00%
TransCanada's subsidiaries | Northern Border
Capital and operating costs 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%	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		$ 9.0				30.0	$ 25.0
Percentage of capital and operating costs charged		100.00%		100.00%	100.00%	100.00%
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		3.0
TransCanada's subsidiaries | GTN | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged	9.0		8.0				24.0	20.0
Impact on the Partnership's net income attributable to controlling interests	7.0		7.0				21.0	18.0
TransCanada's subsidiaries | Bison
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable	1.0						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				4.0	1.0
Impact on the Partnership's net income attributable to controlling interests	2.0		1.0				4.0	2.0
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				3.0	3.0
Impact on the Partnership's net income attributable to controlling interests	1.0		1.0				3.0	3.0
TransCanada's subsidiaries | Tuscarora Gas Transmission Company
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable									1.0
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 attributable to controlling interests	1.0		1.0				3.0	3.0
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
Amount included in receivables from related party	0.0						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				6.0	6.0
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				$ 11.0	$ 9.0
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	44.00%		62.00%				53.00%	69.00%
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				$ 11.0	$ 9.0
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				4.0	3.0
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	$ 0.0		$ 1.0				$ 1.0	$ 2.0

[1] Recast to consolidate PNGTS for all periods presented (Refer to Notes 2 and 6).