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Stock Plans - ESPP FV Assumptions (Details) - ESPP - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based compensation      
Cash proceeds $ 3.1 $ 2.6 $ 1.2
Number of shares issued 332,096 163,000 83,000
Assumptions      
Weighted average fair value (in dollars per share) $ 4.94 $ 7.09 $ 4.45
Dividend yield (as a percent) 0.00% 0.00% 0.00%
Expected volatility factor (as a percent) 62.00% 36.00% 43.00%
Risk-free interest rate (as a percent) 1.81% 0.99% 0.35%
Expected life (in years) 6 months 6 months 6 months