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FAIR VALUE MEASUREMENTS - Interest Rate Swaps (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2017
Dec. 31, 2016
Interest rate derivatives            
Fair value of derivative asset, net           $ 0.0
Amortization of realized gain (loss) on derivative financial instruments     $ 1.0 $ 1.0 [1]    
Debt and credit facilities $ 2,491.0   2,491.0     1,920.0
Amortization of derivatives loss     1.0 1.0    
Term loan | TC PipeLines, LP 2013 Term Loan Facility due July 2018            
Interest rate derivatives            
Debt and credit facilities 500.0   500.0      
Term loan | TC PipeLines, LP 2013 Term Loan Facility due October 2022            
Interest rate derivatives            
Debt and credit facilities $ 500.0   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 by Partnership (as a percent) 61.71%   61.71%      
Net unamortized loss included in other comprehensive income $ 1.0   $ 1.0     2.0
Amortization of derivatives loss $ 0.0 $ 0.0 $ 1.0 1.0    
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%   2.31%      
Hedges of cash flows | Interest rate swaps            
Interest rate derivatives            
Amortization of realized gain (loss) on derivative financial instruments $ 0.0 2.0 $ 1.0 (1.0)    
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 $ 1.0 0.0 $ 2.0    
Hedges of cash flows | Interest rate swaps | Recurring fair value measurement | Level 2            
Interest rate derivatives            
Fair value of derivative asset, net 2.0   2.0      
Designated as hedge | Interest rate swaps | Recurring fair value measurement | Level 2            
Interest rate derivatives            
Fair value of derivative asset, gross $ 2.0   $ 2.0      
Fair value of derivative liability, gross           0.0
Designated as hedge | Hedges of cash flows | Interest rate swaps | Level 2            
Interest rate derivatives            
Fair value of derivative asset, gross           1.0
Fair value of derivative liability, gross           $ 1.0
[1] Recast to consolidate PNGTS for all periods presented (Refer to Notes 2 and 6).