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DEBT AND CREDIT FACILITIES - Amounts Outstanding and Description of Terms (Details)
$ in Millions
6 Months Ended 12 Months Ended
Apr. 05, 2018
USD ($)
Aug. 21, 2017
Jun. 30, 2018
USD ($)
entity
Dec. 31, 2017
USD ($)
Credit facilities, short-term loan facility and long-term debt        
Debt and credit facilities     $ 2,320.0 $ 2,415.0
Less: unamortized debt issuance costs and debt discount     12.0 12.0
Less: current portion     36.0 51.0
Long-term debt     $ 2,272.0 2,352.0
Senior Credit Facility due in 2021 and the Term Loan Facilities due in 2020 and 2022 | Minimum        
Credit facilities, short-term loan facility and long-term debt        
Leverage ratio, actual (as a percent)     4.28%  
Senior Credit Facility due in 2021 and the Term Loan Facilities due in 2020 and 2022 | Debt agreement covenants, initial period after occurrence of acquisition        
Credit facilities, short-term loan facility and long-term debt        
Additional period immediately following the fiscal quarter in which a specified material acquisition occurs     6 months  
Senior Credit Facility due in 2021 and the Term Loan Facilities due in 2020 and 2022 | Debt agreement covenants, initial period after occurrence of acquisition | Minimum        
Credit facilities, short-term loan facility and long-term debt        
Number of acquisitions | entity     1  
Senior Credit Facility due in 2021 and the Term Loan Facilities due in 2020 and 2022 | Debt agreement covenants, initial period after occurrence of acquisition | Maximum        
Credit facilities, short-term loan facility and long-term debt        
Leverage ratio, covenant (as a percent)     550.00%  
Senior Credit Facility due in 2021 and the Term Loan Facilities due in 2020 and 2022 | Debt agreement covenants, periods subsequent to initial period after occurrence of acquisition | Maximum        
Credit facilities, short-term loan facility and long-term debt        
Leverage ratio, covenant (as a percent)     500.00%  
Revolving credit facility | TC PipeLines, LP Senior Credit Facility due 2021        
Credit facilities, short-term loan facility and long-term debt        
Debt and credit facilities     $ 140.0 $ 185.0
Weighted average interest rate (as a percent)     2.99% 2.41%
Maximum borrowing capacity     $ 500.0  
Amount outstanding under credit facility     140.0 $ 185.0
Remaining borrowing capacity     $ 360.0  
Revolving credit facility | TC PipeLines, LP Senior Credit Facility due 2021 | LIBOR        
Credit facilities, short-term loan facility and long-term debt        
Debt interest rate, at period end (as a percent)     3.24% 2.62%
Term loan | TC PipeLines, LP 2013 Term Loan Facility due 2022        
Credit facilities, short-term loan facility and long-term debt        
Debt and credit facilities     $ 500.0 $ 500.0
Weighted average interest rate (as a percent)     3.02% 2.33%
Term loan | TC PipeLines, LP 2013 Term Loan Facility due 2022 | LIBOR borrowings | LIBOR        
Credit facilities, short-term loan facility and long-term debt        
Debt interest rate, at period end (as a percent)     3.24% 2.62%
Term loan | TC PipeLines, LP 2013 Term Loan Facility due 2022 | LIBOR borrowings | LIBOR | Hedges of cash flows | Interest rate swaps        
Credit facilities, short-term loan facility and long-term debt        
Weighted average interest rate (as a percent)     2.31% 2.31%
Term loan | TC PipeLines, LP 2015 Term Loan Facility due 2020        
Credit facilities, short-term loan facility and long-term debt        
Debt and credit facilities     $ 170.0 $ 170.0
Weighted average interest rate (as a percent)     2.91% 2.22%
Term loan | TC PipeLines, LP 2015 Term Loan Facility due 2020 | LIBOR        
Credit facilities, short-term loan facility and long-term debt        
Debt interest rate, at period end (as a percent)     3.13% 2.51%
Unsecured debt | 4.65% Senior Notes due 2021        
Credit facilities, short-term loan facility and long-term debt        
Stated interest rate (as a percent)     4.65% 4.65%
Debt and credit facilities     $ 350.0 $ 350.0
Weighted average interest rate (as a percent)     4.65% 4.65%
Unsecured debt | TC PipeLines, LP 4.375% Senior Notes due 2025        
Credit facilities, short-term loan facility and long-term debt        
Stated interest rate (as a percent)     4.375% 4.375%
Debt and credit facilities     $ 350.0 $ 350.0
Weighted average interest rate (as a percent)     4.375% 4.375%
Unsecured debt | TC PipeLines, LP 3.90% Senior Notes due 2027        
Credit facilities, short-term loan facility and long-term debt        
Stated interest rate (as a percent)     3.90% 3.90%
Debt and credit facilities     $ 500.0 $ 500.0
Weighted average interest rate (as a percent)     3.90% 3.90%
Unsecured debt | GTN 5.29% Senior Notes due 2020        
Credit facilities, short-term loan facility and long-term debt        
Stated interest rate (as a percent)     5.29% 5.29%
Debt and credit facilities     $ 100.0 $ 100.0
Weighted average interest rate (as a percent)     5.29% 5.29%
Unsecured debt | GTN 5.69% Senior Notes due 2035        
Credit facilities, short-term loan facility and long-term debt        
Stated interest rate (as a percent)     5.69% 5.69%
Debt and credit facilities     $ 150.0 $ 150.0
Weighted average interest rate (as a percent)     5.69% 5.69%
Unsecured debt | Term Loan Facility due 2019        
Credit facilities, short-term loan facility and long-term debt        
Debt and credit facilities     $ 35.0 $ 55.0
Weighted average interest rate (as a percent)     2.71% 2.02%
Unsecured debt | Tuscarora Term Loan due 2020        
Credit facilities, short-term loan facility and long-term debt        
Debt and credit facilities     $ 25.0 $ 25.0
Weighted average interest rate (as a percent)     2.87% 2.27%
Unsecured debt | Tuscarora Term Loan due 2020 | LIBOR        
Credit facilities, short-term loan facility and long-term debt        
Debt interest rate, at period end (as a percent)     3.11% 2.49%
Secured debt | PNGTS 5.90% Senior Secured Notes due 2018        
Credit facilities, short-term loan facility and long-term debt        
Stated interest rate (as a percent)     5.90% 5.90%
Debt and credit facilities       $ 30.0
Weighted average interest rate (as a percent)       5.90%
GTN | Unsecured debt | Senior Notes and Term Loan Facility due 2019        
Credit facilities, short-term loan facility and long-term debt        
Debt interest rate, at period end (as a percent)     2.93% 2.31%
Percentage of debt to total capitalization, actual     42.30%  
GTN | Unsecured debt | Senior Notes and Term Loan Facility due 2019 | Maximum        
Credit facilities, short-term loan facility and long-term debt        
Percentage of debt to total capitalization, covenant     70.00%  
Portland Natural Gas Transmission System | PNGTS 5.90% Senior Secured Notes due 2018        
Credit facilities, short-term loan facility and long-term debt        
Payment of principal amount on secured notes       $ 5.8
Portland Natural Gas Transmission System | Secured debt | PNGTS Revolving Credit Facility due 2023 | Maximum        
Credit facilities, short-term loan facility and long-term debt        
Leverage ratio, covenant (as a percent) 500.00%      
Portland Natural Gas Transmission System | Secured debt | PNGTS Revolving Credit Facility due 2023 | LIBOR        
Credit facilities, short-term loan facility and long-term debt        
Maximum borrowing capacity $ 125.0      
Tuscarora Gas Transmission Company | Unsecured Term Loan Facility | Tuscarora Term Loan due 2020 | Minimum        
Credit facilities, short-term loan facility and long-term debt        
Debt service coverage, covenant (as a percent)   300.00%    
Debt service coverage, actual (as a percent)     11.20%