FAIR VALUE MEASUREMENTS - Financial Assets and Liability Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair value measurements
Fair value	$ 78,129	$ 67,686
Fair Value, Measurements, Recurring | Level 1
Fair value measurements
Fair value	50,938	24,528
Fair Value, Measurements, Recurring | Level 1 | Physical fixed price derivative contracts
Fair value measurements
Financial assets	0	0
Financial liabilities	0	0
Fair Value, Measurements, Recurring | Level 1 | Physical index derivative contracts
Fair value measurements
Financial assets	0	0
Financial liabilities	0	0
Fair Value, Measurements, Recurring | Level 1 | Futures contracts for refined petroleum products | Futures Contracts
Fair value measurements
Financial assets	51,641	27,143
Financial liabilities	(703)	(2,615)
Fair Value, Measurements, Recurring | Level 2
Fair value measurements
Fair value	27,191	43,158
Fair Value, Measurements, Recurring | Level 2 | Physical fixed price derivative contracts
Fair value measurements
Financial assets	27,676	44,912
Financial liabilities	(456)	(1,495)
Fair Value, Measurements, Recurring | Level 2 | Physical index derivative contracts
Fair value measurements
Financial assets	25	53
Financial liabilities	(54)	(312)
Fair Value, Measurements, Recurring | Level 2 | Futures contracts for refined petroleum products | Futures Contracts
Fair value measurements
Financial assets	0	0
Financial liabilities	$ 0	$ 0