NET INCOME (LOSS) PER COMMON UNIT - Determination of Net Income (Loss) per Common Unit (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income (loss) per common unit
Net income (loss) attributable to controlling interests									$ 284	$ 280	$ (182)
Amounts attributable to the Class B units									0	(8)	(13)
Net income (loss) allocable to the General Partner and common units									284	272	(195)
Amounts attributable to General Partner's two percent interest									(6)	(5)	4
Common units									$ 278	$ 267	$ (191)
Weighted average common units outstanding – basic and diluted (in units)									71,300,000	71,300,000	71,300,000
Net income (loss) per common unit - basic and diluted (in dollars per unit)	$ 1.01	$ 0.90	$ 0.78	$ 1.21	$ 0.95	$ 0.76	$ 0.75	$ 1.28	$ 3.90	$ 3.74	$ (2.68)
Common units
Net income (loss) per common unit
Common units									$ 278	$ 267	$ (191)
Weighted average common units outstanding – basic and diluted (in units)									71,300,000	71,300,000	71,300,000
Net income (loss) per common unit - basic and diluted (in dollars per unit)									$ 3.90	$ 3.74	$ (2.68)
TC Pipelines, LP
Net income (loss) per common unit
General partner interest									(2.00%)
General Partner | TC Pipelines, LP | Common units
Net income (loss) per common unit
General partner interest									(2.00%)	(2.00%)	(2.00%)