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Summary of significant accounting policies (Details 5)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Stock options and warrants
     
Assumptions used to estimate fair market value of each stock based award granted      
Expected volatility, low end of the range (as a percent) 40.74% 45.13% 27.80%
Expected volatility, high end of the range (as a percent) 42.41% 47.36% 36.68%
Risk-free interest rate, low end of the range (as a percent) 0.94% 0.91% 1.58%
Risk-free interest rate, high end of the range (as a percent) 1.77% 1.39% 2.72%
Stock options and warrants | Minimum
     
Assumptions used to estimate fair market value of each stock based award granted      
Expected life 5 years 6 months 4 days 5 years 3 months 5 years
Stock options and warrants | Maximum
     
Assumptions used to estimate fair market value of each stock based award granted      
Expected life 5 years 11 months 23 days 6 years 6 years 3 months 25 days
Stock options | 2006 Equity Incentive Plan
     
Stock-based compensation expense      
Percentage of awards that will immediately vest if outstanding awards are not assumed or replaced by successor 100.00%    
Stock options | 2012 Equity Incentive Plan
     
Stock-based compensation expense      
Percentage of awards that will immediately vest if outstanding awards are not assumed or replaced by successor 100.00%    
Stock options | Employees
     
Stock-based compensation expense      
Vesting period 3 years    
Expiration term 10 years    
Stock options | Employees | Each 12 month anniversary
     
Stock-based compensation expense      
Vesting rights percentage 33.00%    
Stock options | Director
     
Stock-based compensation expense      
Expiration term 10 years    
Stock options | Director | Initial grant
     
Stock-based compensation expense      
Vesting period 3 years    
Stock options | Director | Initial grant | 12 month anniversary
     
Stock-based compensation expense      
Vesting rights percentage 33.00%    
Stock options | Director | Initial grant | Each month thereafter
     
Stock-based compensation expense      
Vesting rights percentage per month 2.77%    
Stock options | Director | Annual grant | Each month
     
Stock-based compensation expense      
Vesting rights percentage per month 8.33%    
Warrants
     
Stock-based compensation expense      
Expiration term 5 years