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DEBT AND CREDIT FACILITIES - Narrative (Details) - USD ($)
12 Months Ended
Oct. 08, 2020
Jun. 01, 2020
Jun. 26, 2019
Sep. 29, 2017
May 25, 2017
Dec. 31, 2020
Dec. 31, 2019
Jul. 23, 2020
Dec. 19, 2018
Apr. 05, 2018
Iroquois                    
Credit facilities, short-term loan facility and long-term debt                    
Ownership interest         49.34%          
PNGTS                    
Credit facilities, short-term loan facility and long-term debt                    
Ownership interest         11.81%          
2013 Term Loan Facility                    
Credit facilities, short-term loan facility and long-term debt                    
Repayments of secured debt     $ 50,000,000              
Unwind rate of interest rate swap (as a percent)     2.81%              
TC PipeLines, LP Senior Credit Facility due 2021                    
Credit facilities, short-term loan facility and long-term debt                    
Maximum borrowing capacity           $ 500,000,000        
Amount outstanding under credit facility           0        
Remaining borrowing capacity           500,000,000        
Increase in credit facility           500,000,000        
Debt and credit facilities           $ 0 $ 0      
TC PipeLines, LP 2013 Term Loan Facility due 2022                    
Credit facilities, short-term loan facility and long-term debt                    
Effective interest rate (as a percent)           1.40% 2.94%      
Leverage ratio, actual (as a percent)           3.85%        
Debt and credit facilities           $ 450,000,000 $ 450,000,000      
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Fiscal quarter and the two following fiscal quarters in which one or more acquisitions has been executed                    
Credit facilities, short-term loan facility and long-term debt                    
Leverage ratio, actual (as a percent)           5.50%        
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Debt agreement covenants, periods subsequent to initial period after occurrence of acquisition                    
Credit facilities, short-term loan facility and long-term debt                    
Leverage ratio, actual (as a percent)           5.00%        
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Hedges of cash flows | Interest rate swaps                    
Credit facilities, short-term loan facility and long-term debt                    
Effective interest rate (as a percent)           3.26% 3.26%      
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Federal funds rate                    
Credit facilities, short-term loan facility and long-term debt                    
Basis spread on variable rate (as a percent)       0.50%            
TC PipeLines, LP 2013 Term Loan Facility due 2022 | LIBOR                    
Credit facilities, short-term loan facility and long-term debt                    
Basis spread on variable rate (as a percent)       1.00%            
TC PipeLines, LP 2013 Term Loan Facility due 2022 | LIBOR | Minimum                    
Credit facilities, short-term loan facility and long-term debt                    
Basis spread on variable rate (as a percent)       1.125%            
TC PipeLines, LP 2013 Term Loan Facility due 2022 | LIBOR | Maximum                    
Credit facilities, short-term loan facility and long-term debt                    
Basis spread on variable rate (as a percent)       2.00%            
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Base rate | Minimum                    
Credit facilities, short-term loan facility and long-term debt                    
Basis spread on variable rate (as a percent)       0.125%            
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Base rate | Maximum                    
Credit facilities, short-term loan facility and long-term debt                    
Basis spread on variable rate (as a percent)       1.00%            
TC PipeLines, LP 3.90% Senior Notes due 2027                    
Credit facilities, short-term loan facility and long-term debt                    
Amount of debt         $ 500,000,000          
Stated interest rate         3.90% 3.90% 3.90%      
Net proceeds         $ 497,000,000          
Debt and credit facilities           $ 500,000,000 $ 500,000,000      
PNGTS Senior Notes due 2030 | PNGTS | Senior notes                    
Credit facilities, short-term loan facility and long-term debt                    
Debt instrument, term 10 years                  
Amount of debt $ 125,000,000                  
Stated interest rate 2.84%                  
GTN 5.29% Senior Notes due 2020                    
Credit facilities, short-term loan facility and long-term debt                    
Stated interest rate           5.29% 5.29%      
Debt and credit facilities           $ 0 $ 100,000,000      
GTN 5.29% Senior Notes due 2020 | GTN | Unsecured debt                    
Credit facilities, short-term loan facility and long-term debt                    
Amount of debt   $ 100,000,000                
Stated interest rate   5.29%                
GTN 3.12% Series A Senior Notes due 2030 | GTN | Secured debt                    
Credit facilities, short-term loan facility and long-term debt                    
Debt instrument, term   10 years                
Amount of debt   $ 175,000,000                
Stated interest rate   3.12%                
GTN Private Shelf Facility | GTN | Secured debt                    
Credit facilities, short-term loan facility and long-term debt                    
Debt instrument, term 3 years                  
Amount of debt   $ 75,000,000                
Percentage of debt to total capitalization, actual           36.80%        
GTN Private Shelf Facility | Maximum | GTN | Secured debt                    
Credit facilities, short-term loan facility and long-term debt                    
Percentage of debt to total capitalization, covenant   65.00%                
GTN 5.69% Senior Notes due 2035                    
Credit facilities, short-term loan facility and long-term debt                    
Stated interest rate           5.69% 5.69%      
Debt and credit facilities           $ 150,000,000 $ 150,000,000      
GTN 5.69% Senior Notes due 2035 | GTN | Unsecured debt                    
Credit facilities, short-term loan facility and long-term debt                    
Percentage of debt to total capitalization, covenant   70.00%                
PNGTS Revolving Credit Facility due 2023                    
Credit facilities, short-term loan facility and long-term debt                    
Maximum borrowing capacity                   $ 125,000,000
Amount outstanding under credit facility           $ 25,000,000        
Leverage ratio, actual (as a percent)           1.99%       5.00%
Interest rate (as a percent)           1.28% 2.99%      
Debt and credit facilities           $ 25,000,000 $ 39,000,000      
PNGTS Private Shelf Facility | PNGTS | Senior notes                    
Credit facilities, short-term loan facility and long-term debt                    
Leverage ratio, actual (as a percent)           199.00%        
Debt instrument, term 3 years                  
Amount of debt $ 125,000,000                  
Ratio of debt to capitalization           0.37        
PNGTS Private Shelf Facility | Maximum | PNGTS | Senior notes                    
Credit facilities, short-term loan facility and long-term debt                    
Leverage ratio, actual (as a percent) 500.00%                  
Percentage of debt to total capitalization, covenant 65.00%                  
Tuscarora Term Loan due 2020                    
Credit facilities, short-term loan facility and long-term debt                    
Debt and credit facilities           $ 23,000,000 $ 23,000,000      
North Baja Term Loan Due 2021                    
Credit facilities, short-term loan facility and long-term debt                    
Effective interest rate (as a percent)           1.23% 2.77%      
Amount of debt                 $ 50,000,000  
Debt and credit facilities           $ 50,000,000 $ 50,000,000      
Percentage of debt to total capitalization, covenant           70.00%        
Percentage of debt to total capitalization, actual           40.80%        
Term Loan due 2021                    
Credit facilities, short-term loan facility and long-term debt                    
Effective interest rate (as a percent)           2.15% 2.82%      
Debt service coverage, covenant (as a percent)           3.00%        
Debt service coverage, actual (as a percent)           31.16%        
Term Loan due 2021 | Tuscarora | Unsecured debt                    
Credit facilities, short-term loan facility and long-term debt                    
Debt and credit facilities               $ 23,000,000