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Stockholders' equity - Repurchases And Offerings (Details) - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended
Jun. 08, 2016
Mar. 16, 2016
Jun. 30, 2016
Jun. 30, 2015
Repurchase and sale of stock        
Repurchase of common shares proceeds     $ 31,817 $ 1,079
Amount of stock repurchased     $ 30,735  
Common Class A | Private Negotiation        
Repurchase and sale of stock        
Stock repurchased 2,800,000      
Share price (in dollars per share) $ 10.50 $ 13.35    
Repurchase of common shares proceeds $ 29,400      
Number of shares repurchased   100,000    
Amount of stock repurchased   $ 1,300