SEGMENT REPORTING - Reconciliation of Adjusted EBITDA (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	$ 56,722	$ 56,458	$ 64,026
Add (deduct):
Amortization of deferred financing costs	866	861	861
Deferred income taxes	(3,971)	(987)	558
Changes in accounts receivable and other assets	815	3,503	2,079
Changes in accounts payable and accrued expenses	(639)	397	(1,917)
Changes in deferred revenue and other liabilities	(196)	(4,562)	(3,113)
Interest expense, net	(11,356)	(9,917)	(9,837)
Benefit from income taxes	2,669	1,929	247
Foreign currency transaction loss (gain)	14	456	750
Other income, net	0	22	76
Non-cash lease items	0	(341)	0
Non-cash contract asset	(205)	0	0
Net cash provided by operating activities	45,129	47,819	53,730
Corporate activities
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	(5,274)	(4,984)	(5,630)
Add (deduct):
Benefit from income taxes	3	177	(183)
Foreign currency transaction loss (gain)	138	428	651
Terminalling services | Operating Segments
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	62,719	59,900	67,843
Fleet services | Operating Segments
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	$ (723)	$ 1,542	$ 1,813