RELATED PARTY TRANSACTIONS (Details) $ in Millions		1 Months Ended		3 Months Ended	4 Months Ended	12 Months Ended
		Sep. 21, 2017 Bcf	Apr. 24, 2017	Dec. 31, 2015 USD ($)	May 03, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)		Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Feb. 28, 2018	Feb. 01, 2018	Jan. 31, 2018	Sep. 01, 2017	Aug. 01, 2017	Jun. 01, 2017	Jan. 31, 2017	Jan. 01, 2016
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable						$ 5	$ 8	[1],[2]
Interest acquired (as a percent)															49.34%
Amount included in receivables from related party						1	2
Provision for rate refund (liability)	[1]								$ (101)
Great Lakes Settlement | NEB
Capital and operating costs charged to the pipeline systems and amount payable
Term of contract			10 years
Termination options beginning			3 years
Total revenue earned						$ 13
Great Lakes Settlement | NEB | Maximum
Capital and operating costs charged to the pipeline systems and amount payable
Transportation capacity per day | Bcf		0.711
Northern Border
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)														50.00%
Portland Natural Gas Transmission System
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)						61.71%
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%
Refund paid to shippers						$ 7
Percentage of refund paid to shippers						86.00%
Estimated revenue sharing provision						$ 40
Provision for rate refund (liability)						$ 8
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%
Iroquois
Capital and operating costs charged to the pipeline systems and amount payable
Interest acquired (as a percent)						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%
General Partner | Reimbursement of costs of services provided
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged						$ 4	3		3
TransCanada's subsidiaries | Great Lakes
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable						3	4
Amount included in receivables from related party						20	19
TransCanada's subsidiaries | Great Lakes | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged						36	30		30
Impact on the Partnership's net income attributable to controlling interests						$ 15	$ 13		$ 13
Percentage of capital and operating costs charged						100.00%	100.00%		100.00%
Amount included in receivables from related party						$ 64	$ 27
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						57.00%	68.00%		71.00%
TransCanada's subsidiaries | Great Lakes | Affiliated rental revenue
Capital and operating costs charged to the pipeline systems and amount payable
Percent of total revenues						1.00%	1.00%		1.00%
TransCanada's subsidiaries | Northern Border
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable						$ 4	$ 4
TransCanada's subsidiaries | Northern Border | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged						$ 43	$ 32		$ 36
Percentage of capital and operating costs charged						100.00%	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	$ 1
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						$ 9	$ 8		$ 8
Percentage of capital and operating costs charged						100.00%	100.00%		100.00%
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									$ 3
TransCanada's subsidiaries | GTN
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable						$ 3	$ 3
TransCanada's subsidiaries | GTN | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged						34	27		30
Impact on the Partnership's net income attributable to controlling interests						$ 29	$ 24		$ 25
Percentage of capital and operating costs charged						100.00%	100.00%		100.00%
TransCanada's subsidiaries | Bison
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable						$ 1	$ 1
TransCanada's subsidiaries | Bison | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged						6	2		$ 4
Impact on the Partnership's net income attributable to controlling interests						$ 6	$ 3		$ 4
Percentage of capital and operating costs charged						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
Net amounts payable							$ 1
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						$ 4	4		$ 5
Impact on the Partnership's net income attributable to controlling interests						4	4		5
TransCanada's subsidiaries | Tuscarora Gas Transmission Company
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable							1
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						4	5		4
Impact on the Partnership's net income attributable to controlling interests						4	4		4
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						16	12		14
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						5	5		$ 5
TransCanada's subsidiaries | Portland Natural Gas Transmission System | Transportation contracts
Capital and operating costs charged to the pipeline systems and amount payable
Amount included in receivables from related party						0
Revenues from related party						1	$ 2
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					$ 9
Deferred revenue recognized				$ 23
Affiliates | Portland Natural Gas Transmission System | Construction of facilities
Capital and operating costs charged to the pipeline systems and amount payable
Development expenses incurred by affiliates						$ 3

[1] Recast to consolidate PNGTS (Refer to Notes 2 and 7).

[2] Recast to consolidate PNGTS (Refer to Notes 2 and 7).