NET INCOME PER COMMON UNIT - Schedule of net income per common unit (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net income per common unit
Net income attributable to controlling interests	$ 57	$ 55	$ 145	$ 148
Net income attributable to the General Partner	(1)	(1)	(3)	(3)
Net income attributable to common units	$ 56	$ 54	$ 142	$ 145
Weighted average common units outstanding — basic and diluted (millions)	71.3	71.3	71.3	71.3
Net income per common unit - basic and diluted (in dollars per unit)	$ 0.78	$ 0.75	$ 1.99	$ 2.03