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

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Vessels, net and Deferred revenue (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) $ / item	Jun. 30, 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	$ 18,271	$ 16,450	$ 36,348	$ 33,336
Accrual related to estimated customer claims	532		532		$ 345
Vessels
Vessels, net
Depreciation and amortization	$ 16,697	$ 15,246	$ 33,185	$ 30,919