PARTNERS' CAPITAL - Narrative (Details) $ / shares in Units, $ in Millions		3 Months Ended		12 Months Ended
	Apr. 26, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) Transaction $ / shares shares	Mar. 31, 2017 $ / shares	Dec. 31, 2017 USD ($) $ / shares shares
Partners Capital [Line Items]
Preferred units issued | shares		300,000		300,000
Per-unit annual distributions to unitholders (in dollars per unit)		$ 0.575	$ 0.575
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	May 15, 2018
Cash distribution date of record	May 08, 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
Amount of stock sold (in shares) | shares		0
Aggregate offering price | $		$ 132.3
Transactions under ATM Program | Transaction		0
General Partner
Partners Capital [Line Items]
Percentage interest in distributions (maximum)		50.00%