PARTNERS' CAPITAL - Narrative (Details) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 25, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) Transaction $ / shares shares	Sep. 30, 2017 $ / shares	Sep. 30, 2018 USD ($) Transaction $ / shares shares	Sep. 30, 2017 $ / shares	Dec. 31, 2017 USD ($) $ / shares shares
Partners Capital [Line Items]
Preferred units issued | shares		300,000		300,000		300,000
Per-unit annual distributions to unitholders (in dollars per unit)		$ 0.575	$ 0.575	$ 1.725	$ 1.725
Surplus in excess (in dollars per share)				$ 0.46
Subsequent Event
Partners Capital [Line Items]
Per-unit annual distributions to unitholders (in dollars per unit)	$ 0.575
Cash paid to unit holders | $	$ 45.2
Cash distribution date	Nov. 14, 2018
Cash distribution date of record	Nov. 07, 2018
Series A Preferred Units
Partners Capital [Line Items]
Preferred units issued | shares						300,000
Interest in partnership per unit						$ 1,000
Common | Public Offering
Partners Capital [Line Items]
Aggregate offering price | $						$ 150.0
Transactions under ATM Program | Transaction		0		0
Aggregate offering price | $		$ 132.3		$ 132.3
General Partner
Partners Capital [Line Items]
Percentage interest in distributions (maximum)				50.00%