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Fixed assets - Additional Information (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Property Plant And Equipment    
Paid advances relating to the construction of exhaust gas cleaning systems and ballast water treatment systems $ 2,910 $ 0
Vessels' Improvements    
Property Plant And Equipment    
Vessel improvement costs capitalized $ 165 $ 257