PARTNERS' CAPITAL AND DISTRIBUTIONS - Narrative (Details) $ / shares in Units, bbl in Millions, $ in Millions	1 Months Ended					12 Months Ended			33 Months Ended
	Sep. 30, 2014 USD ($) $ / shares shares	Aug. 31, 2014 USD ($) $ / shares shares	Oct. 31, 2013 USD ($) $ / shares shares	May. 31, 2013 USD ($) agreement	Jan. 31, 2013 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($) shares	Sep. 30, 2013 shares bbl	Oct. 31, 2014 shares	Dec. 31, 2012 shares
Limited Partners' Capital Account [Line Items]
Required percentage of limited partners approval for issuing new units						66.67%
Units outstanding (in units)						129,524,000	127,043,000	115,064,000			98,346,000
Limited Partners, units outstanding (in units)						129,523,703	127,043,317
Limited Partners | Equity Distribution Agreements
Limited Partners' Capital Account [Line Items]
Public offering, LP units issued						2,200,000	1,000,000	500,000
Number of equity distribution agreements entity entered | agreement				4
Proceeds from issuance of units | $						$ 161.5	$ 74.5	$ 33.1
Aggregate compensation expense paid to the agents under the Equity Distribution Agreements | $						$ 1.6	$ 0.8	$ 0.4
Limited Partners | Equity Distribution Agreements | Maximum
Limited Partners' Capital Account [Line Items]
Amount of LP Units allowed to be sold under agreement | $				$ 300.0
Limited Partners | Secondary public offering
Limited Partners' Capital Account [Line Items]
Public offering, LP units issued	6,750,000	2,600,000	7,500,000		6,000,000
Proceeds from issuance of units | $	$ 621.0		$ 540.0		$ 362.5
Amount per share or per unit of equity securities issued by non-development stage entity | $ / shares	$ 80.00	$ 76.60	$ 62.61		$ 52.54
Underwriting fees and offering expenses | $	$ 22.0	$ 2.4	$ 19.3		$ 13.3
Aggregate consideration from issuance of units | $		$ 229.0
Limited Partners | Underwriter option
Limited Partners' Capital Account [Line Items]
Public offering, LP units issued			1,100,000		900,000
Units issued (In shares)		400,000
Limited Partners | Underwriter option | Maximum
Limited Partners' Capital Account [Line Items]
Public offering, LP units issued										1,000,000.0
Class B Units
Limited Partners' Capital Account [Line Items]
Trading period immediately preceding the distribution declaration date used to calculate volume-weighted average price of LP Units									10 days
Discount rate used to calculate unit distribution									15.00%
Units outstanding (in units)						0	0	0	8,500,000		7,975,000
Minimum incremental storage capacity, unit conversion threshold (in barrels) | bbl									4
Class B Units | Limited Partners
Limited Partners' Capital Account [Line Items]
Public offering, LP units issued						0
Limited Partners, units outstanding (in units)						0