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FAIR VALUE MEASUREMENTS - Interest Rate Swaps (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Jun. 30, 2018
May 10, 2018
Dec. 31, 2017
Interest rate derivatives              
Change in fair value of interest rate derivative instruments recognized in other comprehensive income (loss)     $ 2.0 $ 1.0      
Debt and credit facilities $ 2,258.0   2,258.0       $ 2,415.0
Amortization of derivatives loss     2.0 1.0      
Term loan | TC PipeLines, LP 2013 Term Loan Facility due 2022              
Interest rate derivatives              
Debt and credit facilities 500.0   500.0       500.0
Portland Natural Gas Transmission System              
Interest rate derivatives              
Payments for derivative instruments     20.9        
Interest acquired (as a percent)           61.71%  
Net unamortized loss included in other comprehensive income             1.0
Amortization of derivatives loss $ 0.0 $ 0.0 $ 1.0 1.0      
Interest rate swaps | Term loan | 2013 Term Loan Facility due 2018              
Interest rate derivatives              
Weighted average fixed interest rate (as a percent) 3.26%   3.26%   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) $ 3.0 0.0 $ 12.0 1.0      
Net realized gain (loss) on reclassified from other comprehensive income (1.0) (1.0) (4.0) 0.0      
Hedges of cash flows | Interest rate swaps | Financial charges and other              
Interest rate derivatives              
Net realized gain (loss) on reclassified from other comprehensive income 0.0 $ 0.0 (2.0) $ 0.0      
Hedges of cash flows | Interest rate swaps | Recurring fair value measurement | Level 2              
Interest rate derivatives              
Fair value of derivative asset, gross             5.0
Fair value of derivative asset, net             $ 5.0
Designated as hedge | Interest rate swaps | Recurring fair value measurement | Level 2              
Interest rate derivatives              
Fair value of derivative asset, gross 17.0   17.0        
Fair value of derivative asset, net $ 17.0   $ 17.0