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Charter arrangements - Shipping revenues (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shipping Revenues [Abstract]      
Time charter revenues $ 100,310 $ 118,997 $ 122,882
Voyage charter revenues 254,742 234,646 241,679
Other shipping revenues   2,366 553
Shipping revenues $ 355,052 $ 356,010 $ 365,114