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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Cumulative effect of new accounting principle in period of adoption $ (2,423,036)    
Revenues 372,970,373 $ 385,945,564  
Retained Earnings (Accumulated Deficit)      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Cumulative effect of new accounting principle in period of adoption (2,423,036)   $ 2,400,000
Deferred Contract Fulfillment Costs      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Deferred revenue 1,300,000 2,300,000  
Revenues 8,100,000    
Voyage revenue      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Revenues 319,753,056 $ 338,540,738  
Accounting Standards Update 2014-09 | Voyage revenue      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Deferred revenue $ 1,700,000   $ 4,700,000