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Segment information (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
USD ($)
Segment
Vessel
Dec. 31, 2019
USD ($)
Vessel
Dec. 31, 2018
USD ($)
Vessel
Operating Segments [Abstract]      
Number of operating segments | Segment 1    
Information About Major Customers [Abstract]      
Number of vessels in operation | Vessel 27 27 27
Revenues $ 691,039 $ 535,068 $ 375,941
Time Charters [Member]      
Information About Major Customers [Abstract]      
Number of vessels in operation | Vessel 18 5 5
Spot Market [Member]      
Information About Major Customers [Abstract]      
Number of vessels in operation | Vessel 9 22 22
Customer One [Member]      
Information About Major Customers [Abstract]      
Revenues $ 78,513 $ 84,067 $ 76,015
Customer Two [Member]      
Information About Major Customers [Abstract]      
Revenues 57,777 79,161 38,408
Customer Three [Member]      
Information About Major Customers [Abstract]      
Revenues 55,371 73,629 19,269
Customer Four [Member]      
Information About Major Customers [Abstract]      
Revenues 53,711 39,477 18,692
Customer Five [Member]      
Information About Major Customers [Abstract]      
Revenues $ 37,670 $ 34,786 $ 18,679