SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Vessels, net (Details) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) $ / item	Sep. 30, 2017 USD ($)
Vessels, net
Estimated useful life			25 years
Depreciation and amortization	$ 17,269	$ 17,836	$ 50,605	$ 54,194
Estimated scrap value (in dollars per lightweight ton) \| $ / item			310
Vessels
Vessels, net
Depreciation and amortization	$ 16,013	$ 16,575	$ 46,933	$ 50,173

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