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Financial instruments - Classes of Financial Instruments (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Classes Of Financial Instruments [Abstract]        
Cash and cash equivalents $ 77,292 $ 109,295 $ 166,775 $ 166,684
Accounts receivable and accrued revenues 42,212 34,461    
Total financial assets 119,504 143,756    
Financial liabilities        
Accounts payable and accrued expenses 17,427 12,378    
Derivative financial liabilities, current 545 2,257    
Current portion long-term debt 65,053 57,521    
Long-term debt 721,151 643,974    
Derivative financial liabilities, non-current 0 442    
Total $ 804,177 $ 716,572