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SUBSEQUENT EVENTS - Distributions (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Nov. 30, 2017
Nov. 01, 2017
Oct. 31, 2017
Oct. 24, 2017
Oct. 23, 2017
Oct. 19, 2017
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Sep. 01, 2017
Jun. 01, 2017
Distributions                        
Partnership distribution [1]                 $ 235.0      
Partnership's share of distributions                 106.0 $ 125.0 [2]    
Return on investment distribution classified as investing activities                 $ 3.0      
Northern Border                        
Distributions                        
Ownership interest (as a percent)     50.00%       50.00%   50.00%   50.00%  
Great Lakes                        
Distributions                        
Ownership interest (as a percent)   46.45%         46.45%   46.45%      
Portland Natural Gas Transmission System                        
Distributions                        
Ownership interest (as a percent)             11.81% 49.90% 11.81% 49.90%   11.81%
Iroquois                        
Distributions                        
Ownership interest (as a percent)   49.34%                    
Portland Natural Gas Transmission System                        
Distributions                        
Ownership interest (as a percent)                       11.81%
Subsequent Events                        
Distributions                        
Total cash distribution $ 75.0                      
Subsequent Events | Distribution declared | Northern Border                        
Distributions                        
Partnership distribution     $ 14.0                  
Subsequent Events | Distribution declared | Great Lakes                        
Distributions                        
Partnership distribution           $ 2.0            
Subsequent Events | Distribution declared | Iroquois                        
Distributions                        
Partnership distribution         $ 28.0              
Subsequent Events | Cash Distribution Paid | Northern Border                        
Distributions                        
Partnership's share of distributions     $ 7.0                  
Subsequent Events | Cash Distribution Paid | Great Lakes                        
Distributions                        
Partnership's share of distributions   $ 1.0                    
Subsequent Events | Cash Distribution Paid | Iroquois                        
Distributions                        
Partnership's share of distributions   14.0                    
Return on investment distribution classified as investing activities   $ 2.6                    
General Partner                        
Distributions                        
Partnership distribution [1]                 $ 12.0      
TC PipeLines GP, Inc. | Subsequent Events                        
Distributions                        
General Partner cash distributions       $ 5.0                
Ownership interest (as a percent)       2.00%                
Total distribution for General Partner interest       $ 2.0                
TC PipeLines GP, Inc. | General Partner                        
Distributions                        
Total distribution for General Partner interest             $ 2.0 $ 1.0 4.0 $ 3.0    
TC PipeLines GP, Inc. | General Partner | Subsequent Events | Distribution declared                        
Distributions                        
Distribution declared for IDRs       $ 3.0                
Common Units                        
Distributions                        
Total cash distribution             $ 74.0 $ 65.0 $ 210.0 $ 184.0    
Number of units             69,600,000 66,600,000 [2] 69,600,000 66,600,000 [2]    
Common Units | Subsequent Events                        
Distributions                        
Per Unit Distribution, declared (in dollars per unit)       $ 1.00                
Limited partners, Distribution declared       $ 70.0                
Common Units | Limited Partners                        
Distributions                        
Partnership distribution [1]                 $ 198.0      
Common Units | TC PipeLines GP, Inc. | Subsequent Events                        
Distributions                        
Limited Partners, Distributions paid 6.0                      
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 $ 11.0                      
Common Units | TransCanada | TC PipeLines GP, Inc.                        
Distributions                        
Number of units       11,287,725                
[1] Recast to consolidate PNGTS for all periods presented (Refer to Notes 2 and 6).
[2] Recast to consolidate PNGTS for all periods presented (Refer to Notes 2 and 6).