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Stock incentive plans - Plans (Details) - shares
Jan. 01, 2018
Jun. 30, 2018
Dec. 31, 2017
2011 Plan      
Stock incentive plans      
Annual percentage increase alternative, increase in common stock reserved for issuance as a percentage of common stock outstanding 4.50%    
Annual fixed increase alternative, increase in shares of common stock reserved for issuance (in shares) 3,000,000    
Stock options      
Stock incentive plans      
Number of options outstanding   541,000 1,283,000
Stock options | 2011 Plan      
Stock incentive plans      
Number of options outstanding   425,000  
RSUs | 2011 Plan      
Stock incentive plans      
RSUs outstanding   3,371,000