Stockholders' Equity (Schedule of Fair Value of Employee Stock Purchase Plan) (Details) - ESPP [Member] |
12 Months Ended | ||
---|---|---|---|
Dec. 31, 2020 |
Dec. 31, 2019 |
Dec. 31, 2018 |
|
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] | |||
Expected volatility | |||
Risk-free interest rate | |||
Risk-free interest rate, minimum | 0.10% | 1.30% | |
Risk-free interest rate, maximum | 1.00% | 2.70% | |
Dividend yield | |||
Minimum [Member] | |||
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] | |||
Expected term (years) | 6 months | 6 months | |
Expected volatility | 57.00% | 65.00% | |
Maximum [Member] | |||
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] | |||
Expected term (years) | 2 years | 2 years | |
Expected volatility | 71.00% | 67.00% |