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Earnings/ (Loss) per share - Additional information (Details) - USD ($)
$ in Thousands
 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Earnings Per Share, Basic [Abstract]    
Issuance date of Series B preferred stock August 2013  
Issuance Date Of Series C preferred Stock January 2014  
Issuance Date Of Series D preferred Stock May 2015  
Preferred stock dividend rate series B 7.625%  
Preferred stock dividend rate series C 8.50%  
Preferred stock dividend rate series D 8.75%  
Dividends paid and accrued during period $ 7,313 $ 5,719