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RELATED PARTY TRANSACTIONS (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2018
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2017
Capital and operating costs charged to the pipeline systems and amount payable              
Net amounts payable $ 6.0           $ 5.0
Amount included in receivables from related party 3.0           1.0
Great Lakes              
Capital and operating costs charged to the pipeline systems and amount payable              
Estimated revenue sharing provision             40.0
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        
TransCanada's subsidiaries | Great Lakes              
Capital and operating costs charged to the pipeline systems and amount payable              
Net amounts payable 3.0           3.0
Percentage of capital and operating costs charged   100.00%   100.00% 100.00% 100.00%  
Amount included in receivables from related party 10.0           20.0
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   $ 8.0        
Percentage of capital and operating costs charged 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
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 9.0   $ 10.0        
Percentage of capital and operating costs charged   100.00%   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 1.0           1.0
Amount included in receivables from related party 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        
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
TransCanada's subsidiaries | GTN | Capital and operating costs              
Capital and operating costs charged to the pipeline systems and amount payable              
Costs charged 8.0   7.0        
Impact on the Partnership's net income attributable to controlling interests 8.0   7.0        
TransCanada's subsidiaries | Bison              
Capital and operating costs charged to the pipeline systems and amount payable              
Net amounts payable 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        
Impact on the Partnership's net income attributable to controlling interests 2.0   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        
Impact on the Partnership's net income attributable to controlling interests 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        
Impact on the Partnership's net income attributable to controlling interests 1.0   1.0        
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        
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 68.00%   67.00%        
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        
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        
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 1.0           $ 0.0
Affiliates | Portland Natural Gas Transmission System              
Capital and operating costs charged to the pipeline systems and amount payable              
Net amounts payable $ 5.0