Quarterly report pursuant to Section 13 or 15(d)

Stock Plans - Fair value of stock options (Details)

v3.19.2
Stock Plans - Fair value of stock options (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Stock Plans        
Weighted-average risk-free interest rate 2.10% 2.90% 2.20% 2.70%
Expected dividend payments $ 0 $ 0 $ 0 $ 0
Expected holding period (years) 5 years 10 months 24 days [1] 6 years 4 months 24 days [1] 5 years 4 months 24 days 6 years 4 months 24 days
Weighted-average volatility factor 0.93% [2] 0.91% [2] 0.94% 0.89%
Estimated forfeiture rates for options granted 21.00% [3] 25.00% [3] 21.00% 26.00%
[1] Expected holding period is based on historical experience of similar awards, giving consideration to the contractual terms of the stock-based awards, vesting schedules and the expectations of future employee behavior.
[2] Weighted average volatility is based on the historical volatility of our common stock.
[3] Estimated forfeiture rates are based on historical data.