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Summary of significant accounting policies - Vesting requirements & Fair Value Inputs (Details) - Stock options
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Assumptions used to estimate fair market value of each stock based award granted      
Expected volatility, low end of the range (as a percent) 35.11% 38.48% 40.74%
Expected volatility, high end of the range (as a percent) 39.57% 41.98% 42.41%
Risk-free interest rate, low end of the range (as a percent) 1.52% 1.77% 0.94%
Risk-free interest rate, high end of the range (as a percent) 1.94% 2.02% 1.77%
2006 Equity Incentive Plan      
Stock-based expense      
Percentage of awards that will immediately vest if outstanding awards are not assumed or replaced by successor   100.00%  
2012 Equity Incentive Plan      
Stock-based expense      
Percentage of awards that will immediately vest if outstanding awards are not assumed or replaced by successor   100.00%  
Minimum      
Assumptions used to estimate fair market value of each stock based award granted      
Expected life 5 years 4 months 28 days 5 years 10 months 17 days 5 years 6 months 4 days
Maximum      
Assumptions used to estimate fair market value of each stock based award granted      
Expected life 6 years 6 years 7 days 5 years 11 months 23 days
Employees      
Stock-based expense      
Vesting period 3 years    
Expiration term 10 years    
Employees | Vesting period each 12-month anniversary      
Stock-based expense      
Vesting rights percentage 33.00%    
Directors      
Stock-based expense      
Expiration term 10 years    
Directors | Initial grant      
Stock-based expense      
Vesting period 3 years    
Directors | Initial grant | Vesting period 12-month anniversary      
Stock-based expense      
Vesting rights percentage 33.00%    
Directors | Initial grant | Vesting period each month thereafter      
Stock-based expense      
Vesting rights percentage per month 2.77%    
Directors | Annual grant | Vesting period each month      
Stock-based expense      
Vesting rights percentage per month 8.33%