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
	Mar. 31, 2020	Mar. 31, 2019
Net income per common unit
Net income attributable to controlling interests	$ 88	$ 93
Net income attributable to the General Partner	(2)	(2)
Net income attributable to common units	$ 86	$ 91
Weighted average common units outstanding — basic and diluted (millions)	71.3	71.3
Net income per common unit - basic and diluted (in dollars per unit)	$ 1.21	$ 1.28