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Summary of Significant Accounting Policies - Leases (Details)
$ in Thousands
 6 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2020
USD ($)
lease
vessel
Property, Plant and Equipment [Line Items]    
Number of vessels chartered to customers | vessel   12
Lease payments | $   $ 2,404
Time charter, term to completion 15 years 30 days
Number of noncancelable office leases | lease   2
Minimum    
Property, Plant and Equipment [Line Items]    
Vessel lease term   20 days
Maximum    
Property, Plant and Equipment [Line Items]    
Vessel lease term   83 days