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Significant accounting policies (Details)
 12 Months Ended
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Jan. 01, 2019
USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Voyage revenues $ 821,365,000 $ 651,561,000 $ 331,976,000    
Provision for doubtful debts $ 1,607,000 722,000 0    
Vessels' depreciation method straight-line basis        
Vessel's useful life 25 years        
Vessel salvage value per LWT $ 300 300      
Revenue recognition method straight-line basis        
Number of reportable segments 1        
Finance Lease, Right-of-Use Asset   1,047,780,000      
Finance Lease, Liability   609,737,000      
Weighted-average discount rate 5.66%        
Operating Lease, Right-of-Use Asset $ 1,216,000 0      
Operating Lease, Liability 1,216,000 0      
Operating Lease, Expense $ 126,813,000 92,896,000 5,325,000    
Operating Lease, Weighted Average Remaining Lease Term 2 years 2 months 1 day        
Net cash provided by / (used in) Operating Activities $ 88,525,000 169,009,000 82,804,000    
Net cash provided by / (used in) Investing Activities (279,837,000) (325,327,000) (127,101,000)    
Charter-in vessels          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Voyage revenues 185,311,000 127,618,000 6,153,000    
Revenue recognized from subleases 15,253,000 18,661,000 3,005,000    
Office rental arrangements          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Operating Lease, Right-of-Use Asset 1,216,000       $ 1,198,000
Operating Lease, Liability $ 1,216,000       $ 1,198,000
Discount rate 4.00%        
Operating Lease, Expense $ 352,000 $ 403,000      
Operating Lease, Weighted Average Remaining Lease Term 3 years 10 months 27 days        
Effect of Standard Adoption | ASU 2016-18          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Net cash provided by / (used in) Operating Activities     1,834,000 $ (216,000)  
Net cash provided by / (used in) Investing Activities     249,000 209,000  
Balances without Standard Adoption | ASU 2016-18          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Net cash provided by / (used in) Operating Activities     80,970,000 (33,448,000)  
Net cash provided by / (used in) Investing Activities     $ (126,852,000) $ (13,216,000)