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FAIR VALUE MEASUREMENTS - Interest Rate Swaps (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Mar. 31, 2017
USD ($)
Interest rate derivatives        
Change in fair value of interest rate derivative instruments recognized in other comprehensive income (loss) $ 1.0 $ 1.0 [1] $ 1.0 [1]  
Amortization of derivatives loss 1.0 1.0 $ 1.0  
Accounts receivable        
Interest rate derivatives        
Maximum counterparty credit exposure $ 0.0      
Number of credit risk customers | item 1      
Term loan | TC PipeLines, LP 2013 Term Loan Facility due 2022        
Interest rate derivatives        
Debt and credit facilities $ 500.0 $ 500.0    
Amount of forward starting swaps hedged       $ 500.0
Average rate of forward starting swaps       3.26%
Portland Natural Gas Transmission System        
Interest rate derivatives        
Payments for derivative instruments $ 20.9      
Interest acquired (as a percent) 61.71% 61.71% 61.71%  
Net unamortized loss included in other comprehensive income $ 1.0 $ 2.0    
Amortization of derivatives loss $ 0.8 0.8 $ 0.8  
Interest rate swaps | Term loan | TC PipeLines, LP 2013 Term Loan Facility due July 2018        
Interest rate derivatives        
Weighted average fixed interest rate (as a percent) 2.31%      
Hedges of cash flows | Interest rate swaps        
Interest rate derivatives        
Change in fair value of interest rate derivative instruments recognized in other comprehensive income (loss) $ 5.0 2.0 0.0  
Hedges of cash flows | Interest rate swaps | Financial charges and other        
Interest rate derivatives        
Net realized loss related to the interest rate swaps 0.0 3.0 $ 2.0  
Hedges of cash flows | Interest rate swaps | Level 2        
Interest rate derivatives        
Fair value of derivative asset, gross 5.0      
Fair value of derivative asset, net 5.0      
Hedges of cash flows | Interest rate swaps | Recurring fair value measurement | Level 2        
Interest rate derivatives        
Fair value of derivative asset, gross   1.0    
Fair value of derivative liability, gross   1.0    
Fair value of derivative asset, net $ 5.0      
Fair value of derivative liability, net   $ 0.0    
[1] Recast to consolidate PNGTS (Refer to Notes 2 and 7).