FAIR VALUE MEASUREMENTS - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair value measurements
Fair value	$ 47,234	$ 33,808
Fair Value, Measurements, Recurring | Level 1
Fair value measurements
Fair value	0	(26,219)
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
Fair value measurements
Financial liabilities	0	(26,219)
Fair Value, Measurements, Recurring | Level 1 | Interest rate derivatives
Fair value measurements
Financial assets	0	0
Fair Value, Measurements, Recurring | Level 2
Fair value measurements
Fair value	47,234	60,027
Fair Value, Measurements, Recurring | Level 2 | Physical fixed-price derivative contracts
Fair value measurements
Financial assets	818	1,352
Financial liabilities	(3,746)	(4,267)
Fair Value, Measurements, Recurring | Level 2 | Physical index derivative contracts
Fair value measurements
Financial assets	226	333
Financial liabilities	(3)	0
Fair Value, Measurements, Recurring | Level 2 | Futures contracts for refined petroleum products
Fair value measurements
Financial liabilities	0	0
Fair Value, Measurements, Recurring | Level 2 | Interest rate derivatives
Fair value measurements
Financial assets	$ 49,939	$ 62,609