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Segment information (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
USD ($)
Segment
Vessel
Dec. 31, 2018
USD ($)
Vessel
Dec. 31, 2017
USD ($)
Vessel
Operating Segments [Abstract]      
Number of operating segments | Segment 1    
Information About Major Customers [Abstract]      
Number of vessels in operation | Vessel 27 27 26
Revenues $ 535,068 $ 375,941 $ 355,052
Time Charters [Member]      
Information About Major Customers [Abstract]      
Number of vessels in operation | Vessel 5 5 6
Spot Market [Member]      
Information About Major Customers [Abstract]      
Number of vessels in operation | Vessel 22 22 20
Customer One [Member]      
Information About Major Customers [Abstract]      
Revenues $ 84,067 $ 76,015 $ 48,192
Customer Two [Member]      
Information About Major Customers [Abstract]      
Revenues 79,161 38,408 39,488
Customer Three [Member]      
Information About Major Customers [Abstract]      
Revenues 73,629 19,269 36,109
Customer Four [Member]      
Information About Major Customers [Abstract]      
Revenues 39,477 18,692 32,252
Customer Five [Member]      
Information About Major Customers [Abstract]      
Revenues $ 34,786 $ 18,679 $ 18,855