NET INCOME PER COMMON UNIT- Terms of Class B Unit Distributions and Determination of Net Income (Loss) per Common Unit (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Net income per common unit
Net income attributable to controlling interests	$ 93	$ 96
Net income attributable to the General Partner	(2)	(2)
Class B units
Net income per common unit
Net income attributable to common units	0	0
Common units
Net income per common unit
Net income attributable to common units	$ 91	$ 94
Weighted average common units outstanding - basic (in units)	71.3	71.2
Weighted average common units outstanding - diluted (in units)	71.3	71.2
Net income (loss) per common unit - basic (in dollars per unit)	$ 1.28	$ 1.32
Net income (loss) per common unit - diluted (in dollars per unit)	$ 1.28	$ 1.32
GTN | Class B units | TC Energy | Distributions
Distributions
Percentage applied to GTN's distributable cash flow	30.00%
GTN's distributions for payment to Class B units	$ 20		$ 20	$ 20