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SUBSEQUENT EVENTS - Distributions (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended 9 Months Ended
Nov. 02, 2018
Nov. 01, 2018
Oct. 23, 2018
Oct. 22, 2018
Oct. 17, 2018
Oct. 10, 2018
Oct. 31, 2018
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Mar. 31, 2018
Distributions                          
Partnership distribution                     $ 197    
Partnership's share of distributions                     $ 142 $ 106  
Northern Border                          
Distributions                          
Ownership interest (as a percent)               50.00%     50.00%    
Great Lakes                          
Distributions                          
Ownership interest (as a percent)   46.45%           46.45%     46.45%   46.45%
Subsequent Events                          
Distributions                          
Total cash distribution $ 47                        
Percentage of reduction in cash distributions 35.00%                        
Subsequent Events | Northern Border                          
Distributions                          
Ownership interest (as a percent)             50.00%            
Subsequent Events | Distribution declared | Portland Natural Gas Transmission System                          
Distributions                          
Partnership distribution     $ 8                    
Subsequent Events | Distribution declared | Northern Border                          
Distributions                          
Partnership distribution           $ 15              
Subsequent Events | Distribution declared | Great Lakes                          
Distributions                          
Partnership distribution         $ 22                
Subsequent Events | Cash Distribution Paid | Northern Border                          
Distributions                          
Partnership's share of distributions             $ 7            
Subsequent Events | Cash Distribution Paid | Great Lakes                          
Distributions                          
Partnership's share of distributions   $ 10                      
Subsequent Events | Cash Distribution Paid | Portland Natural Gas Transmission System                          
Distributions                          
Share of distributions to its non-controlling interest owner   $ 3                      
General Partner                          
Distributions                          
Partnership distribution                     $ 7    
TC PipeLines GP, Inc. | Subsequent Events                          
Distributions                          
General Partner cash distributions $ 1                        
Ownership interest (as a percent) 2.00%                        
Common Units                          
Distributions                          
Per Unit Distribution, declared (in dollars per unit)                 $ 1.00        
Total cash distribution               $ 47   $ 74 $ 171 $ 210  
Number of units               71,300,000   69,600,000 71,300,000 69,600,000  
Common Units | Subsequent Events                          
Distributions                          
Per Unit Distribution, declared (in dollars per unit)       $ 0.65                  
Limited partners, Distribution declared $ 46                        
Common Units | Limited Partners                          
Distributions                          
Partnership distribution                     $ 164    
Common Units | TC PipeLines GP, Inc. | Subsequent Events                          
Distributions                          
Limited Partners, Distributions paid $ 4                        
Common Units | TC PipeLines GP, Inc. | Limited Partners                          
Distributions                          
Number of units 5,797,106                        
Common Units | TransCanada | Subsequent Events                          
Distributions                          
Limited Partners, Distributions paid $ 7                        
Common Units | TransCanada | TC PipeLines GP, Inc.                          
Distributions                          
Number of units 11,287,725