RELATED PARTY TRANSACTIONS (Details) - USD ($) $ in Millions						3 Months Ended		6 Months Ended			12 Months Ended
		Aug. 01, 2017	Jul. 31, 2017	Jul. 18, 2017	Jul. 07, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016		Dec. 31, 2016		Jun. 01, 2017
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable						$ 35.0		$ 35.0			$ 8.0	[1]
Amount included in receivables from related party						$ 1.0		1.0			2.0
Partnership distribution	[2]							160.0
Partnership's share of distributions								$ 68.0	$ 95.0	[1]
Great Lakes | Distribution declared | Subsequent Events
Capital and operating costs charged to the pipeline systems and amount payable
Partnership distribution				$ 15.0
Northern Border | Distribution declared | Subsequent Events
Capital and operating costs charged to the pipeline systems and amount payable
Partnership distribution					$ 14.0
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 | Cash Distribution Paid | Subsequent Events
Capital and operating costs charged to the pipeline systems and amount payable
Partnership's share of distributions		$ 7.0
Great Lakes | FERC
Capital and operating costs charged to the pipeline systems and amount payable
Estimated revenue sharing provision						$ 7.0		$ 10.0			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%
Northern Border | Cash Distribution Paid | Subsequent Events
Capital and operating costs charged to the pipeline systems and amount payable
Partnership's share of distributions			$ 7.0
Iroquois
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)						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	$ 2.0	$ 2.0
TransCanada's subsidiaries | Great Lakes
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable						$ 4.0		$ 4.0			4.0
Percentage of capital and operating costs charged						100.00%	100.00%	100.00%	100.00%
Amount included in receivables from related party						$ 1.0		$ 1.0			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						$ 9.0	$ 7.0	$ 17.0	$ 14.0
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						$ 4.0		$ 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						$ 10.0	$ 9.0	$ 20.0	$ 15.0
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	$ 7.0	16.0	$ 13.0
Impact on the Partnership's net income attributable to controlling interests						7.0	6.0	14.0	11.0
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						1.0	1.0	2.0
Impact on the Partnership's net income attributable to controlling interests						1.0	1.0	2.0	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	2.0	2.0
Impact on the Partnership's net income attributable to controlling interests						1.0	1.0	2.0	2.0
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	2.0	2.0
Impact on the Partnership's net income attributable to controlling interests						1.0	1.0	2.0	2.0
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	4.0	4.0
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	$ 7.0	$ 6.0
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						43.00%	64.00%	57.00%	71.00%
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	$ 7.0	$ 6.0
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	$ 2.0	$ 2.0
TransCanada's subsidiaries | Portland Natural Gas Transmission System | Transportation contracts | Total net revenues | Customer concentration risk
Capital and operating costs charged to the pipeline systems and amount payable
Percent of total revenues						2.00%	5.00%	1.00%	3.00%

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

[2] Recast to consolidate PNGTS for all periods presented. See Notes 2 and 6.