RELATED PARTY TRANSACTIONS (Details) $ in Millions		1 Months Ended	3 Months Ended	12 Months Ended
	Nov. 01, 2017	Sep. 21, 2017 Bcf	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				$ 6	$ 5
Amount included in receivables from related party				2	1
General Partner | Reimbursement of costs of services provided
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				4	4	$ 3
TransCanada's subsidiaries | Great Lakes
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				3	3
Amount included in receivables from related party				18	20
TransCanada's subsidiaries | Great Lakes | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				44	36	30
Impact on the Partnership's net income attributable to controlling interests				19	15	$ 13
Amount included in receivables from related party				$ 36	$ 64
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				73.00%	57.00%	68.00%
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%
TransCanada's subsidiaries | Great Lakes | Affiliated rental revenue | Maximum
Capital and operating costs charged to the pipeline systems and amount payable
Percent of total revenues				1.00%
TransCanada's subsidiaries | Northern Border
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				$ 3	$ 4
TransCanada's subsidiaries | Northern Border | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				36	43	$ 32
Impact on the Partnership's net income attributable to controlling interests				16	16	12
TransCanada's subsidiaries | PNGTS
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				1	1
TransCanada's subsidiaries | PNGTS | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				9	9	8
Impact on the Partnership's net income attributable to controlling interests				5	5	5
TransCanada's subsidiaries | PNGTS | 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	1	2
TransCanada's subsidiaries | GTN
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				4	3
TransCanada's subsidiaries | GTN | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				34	34	27
Impact on the Partnership's net income attributable to controlling interests				$ 28	$ 29	$ 24
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	6	$ 2
Impact on the Partnership's net income attributable to controlling interests				6	6	3
TransCanada's subsidiaries | North Baja | Capital and operating costs
Capital and operating costs charged to the pipeline systems and amount payable
Costs charged				4	4	4
Impact on the Partnership's net income attributable to controlling interests				4	4	4
TransCanada's subsidiaries | Tuscarora
Capital and operating costs charged to the pipeline systems and amount payable
Net amounts payable				1
TransCanada's subsidiaries | Tuscarora | 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	$ 4
TransCanada | Great Lakes
Capital and operating costs charged to the pipeline systems and amount payable
Termination options beginning	3 years
TransCanada | Great Lakes | Transportation contracts
Capital and operating costs charged to the pipeline systems and amount payable
Contract term	10 years
Total revenue earned				76	13
TransCanada | PNGTS
Capital and operating costs charged to the pipeline systems and amount payable
Amount included in receivables from related party					0
ANR Pipeline Company | Great Lakes | Transportation contracts
Capital and operating costs charged to the pipeline systems and amount payable
Transportation capacity per day | Bcf		0.711
ANR Pipeline Company | Great Lakes | Transportation contracts
Capital and operating costs charged to the pipeline systems and amount payable
Contract term			15 years
Total contract value			$ 1,300
Affiliates | PNGTS | Construction of facilities
Capital and operating costs charged to the pipeline systems and amount payable
Reimbursement of costs				$ 47	$ 3