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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration Risk (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) customer item	Dec. 31, 2016 customer item	Dec. 31, 2015 customer
Concentration Risk
Number of financial institutions with which the entity maintains its cash and cash equivalents | item	3	4
Cash insured by financial institutions | $	$ 0
Voyage Revenues | Customer Concentration Risk
Concentration Risk
Percentage of revenue earned (as a percent)	100.00%
Number of customers	102	52	52
Major Customers	2	3	3
Concentration risk percentage (as a percent)	10.00%	10.00%	10.00%
Voyage Revenues | Customer Concentration Risk | Swissmarine Services S.A.
Concentration Risk
Concentration risk percentage (as a percent)	15.09%	25.31%	24.37%
Voyage Revenues | Customer Concentration Risk | Clipper Group
Concentration Risk
Concentration risk percentage (as a percent)	10.98%	22.96%	19.09%
Voyage Revenues | Customer Concentration Risk | Pioneer Navigation Ltd
Concentration Risk
Concentration risk percentage (as a percent)		11.11%	13.03%

X

- Definition

Represents the number customers of the entity from which revenue was earned.

+ References

No definition available.

+ Details

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- Definition

Represents the number of financial institutions with which the entity maintains its cash and cash equivalents.

+ References

No definition available.

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- Definition

Percentage of revenue earned from customers.

+ References

No definition available.

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- Definition

Represents the number of external customers from which the entity generated 10 percent or more of revenues.

+ References

No definition available.

+ Details

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X

- Definition

The amount of cash deposited in financial institutions as of the balance sheet date that is insured by the Federal Deposit Insurance Corporation.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13537-108611

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