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Significant accounting policies and recent accounting pronouncements (Details) - USD ($)
$ in Thousands
 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jan. 01, 2019
Dec. 31, 2018
Lessee, Lease, Description [Line Items]        
Voyage revenues $ 324,282 $ 253,661    
Finance Lease, Right-of-Use Asset       $ 1,047,780
Finance Lease, Liability       609,737
Operating Lease, Right-of-Use Asset 1,140     0
Operating Lease, Liability 1,140     $ 0
Charter-in vessels        
Lessee, Lease, Description [Line Items]        
Voyage revenues 78,449 56,093    
Revenue recognized from subleases 11,399 7,402    
Office rental arrangements        
Lessee, Lease, Description [Line Items]        
Operating Lease, Right-of-Use Asset 1,140   $ 1,198  
Operating Lease, Liability 1,140   $ 1,198  
Operating Lease, Expense $ 121 $ 192    
Operating Lease, Weighted Average Remaining Lease Term 4 years 4 months 24 days