Annual report pursuant to Section 13 and 15(d)

Stock Plans (Details 2)

v3.8.0.1
Stock Plans (Details 2) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Goods and Nonemployee Services Transaction [Line Items]      
Weighted-average risk-free interest rate 2.13% 1.53% 1.88%
Expected dividend payments $ 0 $ 0 $ 0
Expected holding period (years) [1] 5 years 10 months 24 days 6 years 6 months 11 days 6 years 5 months 23 days
Weighted-average volatility factor [2] 0.90% 0.92% 1.16%
Estimated forfeiture rates for options granted 27.00% 29.00% 30.00%
[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.