SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Vessels, net and Deferred revenue (Details) $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($) $ / item	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Vessels, net
Estimated useful life	25 years
Estimated scrap value (in dollars per lightweight ton) \| $ / item	310
Depreciation and amortization	$ 18,076	$ 16,886
Accrual related to estimated customer claims	640		$ 345
Vessels
Vessels, net
Depreciation and amortization	$ 16,488	$ 15,673

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Name:	us-gaap_DepreciationDepletionAndAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration