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Stock Plans - Performance Awards Assumptions (Details) - Performance awards with market conditions - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Assumptions      
Weighted average fair value (in dollars per share) $ 10.59 $ 16.45 $ 15.58
Dividend yield (as a percent) 0.00% 0.00% 0.00%
Expected volatility factor (as a percent) 60.27% 53.00% 49.00%
Risk-free interest rate (as a percent) 0.54% 2.37% 2.88%
Expected life (in years) 3 years 2 years 9 months 18 days 3 years