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Share Based Awards - Options assumptions (Details)
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Incentive Stock Options    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Risk-free interest rate 2.10% 1.40%
Expected volatility 41.00% 42.00%
Expected life (in years) 6 years 1 month 6 days 6 years 1 month 6 days
Employees Stock Purchase Plan    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Risk-free interest rate 0.90% 0.60%
Expected volatility 34.00% 39.00%
Expected life (in years) 1 year 3 months 18 days 1 year 3 months 18 days
Minimum | Incentive Stock Options    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected volatility   42.00%
Expected life (in years)   6 years 1 month 6 days
Minimum | Employees Stock Purchase Plan    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Risk-free interest rate   0.60%
Expected volatility 34.00% 39.00%