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Income taxes (Details 2) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Income taxes    
Change in valuation allowance $ 16.3 $ 9.2
Federal    
Operating loss carryforwards    
Net operating loss carryforwards 270.4  
Operating loss carryforwards associated to exercise of stock options and vesting of restricted stock 49.2  
Tax benefit related to operating loss carryforwards 16.7  
State    
Operating loss carryforwards    
Net operating loss carryforwards $ 198.1