SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Vessels, net (Details) $ in Thousands	3 Months Ended
Mar. 31, 2018 USD ($) $ / item	Mar. 31, 2017 USD ($)
Vessels, net
Estimated useful life	25 years
Depreciation and amortization	$ 16,886	$ 18,173
Estimated scrap value (in dollars per lightweight ton) \| $ / item	310
Vessels
Vessels, net
Depreciation and amortization	$ 15,673	$ 16,706

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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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