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CASH DISTRIBUTIONS - General Partner Distribution Incentives (Details) - $ / shares
3 Months Ended 9 Months Ended 12 Months Ended
Apr. 01, 2015
Mar. 31, 2017
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Apr. 25, 2017
Mar. 31, 2016
Common units                  
Schedule of Capitalization, Equity [Line Items]                  
Number of units [1]   68,600,000     67,400,000 64,300,000 63,600,000   64,700,000
Limited Partners | Common units                  
Schedule of Capitalization, Equity [Line Items]                  
Number of units         67,454,831        
Limited Partners | Common units | TC PipeLines GP, Inc.                  
Schedule of Capitalization, Equity [Line Items]                  
Number of units         5,797,106     5,797,106  
General Partner | TC PipeLines GP, Inc.                  
Schedule of Capitalization, Equity [Line Items]                  
General partner interest (as a percent) 2.00% 2.00% 2.00% 2.00% 2.00% 2.00% 2.00%    
Minimum Quarterly Distribution                  
Schedule of Capitalization, Equity [Line Items]                  
Total Quarterly Distribution Per Unit Target Amount (in dollars per unit)         $ 0.45        
Minimum Quarterly Distribution | Limited Partners | Common units                  
Schedule of Capitalization, Equity [Line Items]                  
Marginal Percentage Interest in Distribution, Common Unitholders (as a percent)         98.00%        
Minimum Quarterly Distribution | General Partner                  
Schedule of Capitalization, Equity [Line Items]                  
Marginal Percentage Interest in Distribution, General Partner (includes IDRs)         2.00%        
First Target Distribution | Minimum                  
Schedule of Capitalization, Equity [Line Items]                  
Total Quarterly Distribution Per Unit Target Amount (in dollars per unit)         $ 0.45        
First Target Distribution | Maximum                  
Schedule of Capitalization, Equity [Line Items]                  
Total Quarterly Distribution Per Unit Target Amount (in dollars per unit)         $ 0.81        
First Target Distribution | Limited Partners | Common units                  
Schedule of Capitalization, Equity [Line Items]                  
Marginal Percentage Interest in Distribution, Common Unitholders (as a percent)         98.00%        
First Target Distribution | General Partner                  
Schedule of Capitalization, Equity [Line Items]                  
Marginal Percentage Interest in Distribution, General Partner (includes IDRs)         2.00%        
Second Target Distribution | Minimum                  
Schedule of Capitalization, Equity [Line Items]                  
Total Quarterly Distribution Per Unit Target Amount (in dollars per unit)         $ 0.81        
Second Target Distribution | Maximum                  
Schedule of Capitalization, Equity [Line Items]                  
Total Quarterly Distribution Per Unit Target Amount (in dollars per unit)         $ 0.88        
Second Target Distribution | Limited Partners | Common units                  
Schedule of Capitalization, Equity [Line Items]                  
Marginal Percentage Interest in Distribution, Common Unitholders (as a percent)         85.00%        
Second Target Distribution | General Partner                  
Schedule of Capitalization, Equity [Line Items]                  
Marginal Percentage Interest in Distribution, General Partner (includes IDRs)         15.00%        
Thereafter | Minimum                  
Schedule of Capitalization, Equity [Line Items]                  
Total Quarterly Distribution Per Unit Target Amount (in dollars per unit)         $ 0.88        
Thereafter | Limited Partners | Common units                  
Schedule of Capitalization, Equity [Line Items]                  
Marginal Percentage Interest in Distribution, Common Unitholders (as a percent)         75.00%        
Thereafter | General Partner                  
Schedule of Capitalization, Equity [Line Items]                  
Marginal Percentage Interest in Distribution, General Partner (includes IDRs)         25.00%        
[1] Recast to consolidate PNGTS for all periods presented (Refer to Note 2).