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Stock-based Compensation Expense (Assumption Used to Estimate the Fair Value of Stock-Based Awards) (Details) - shares
3 Months Ended 6 Months Ended
Jul. 31, 2020
Jul. 31, 2019
Jul. 31, 2020
Jul. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected dividends (in percentage)     0.00%  
Performance Stock Unit        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Range of expected volatilities (in percentage)     50.70% 36.30%
Expected dividends (in percentage)     0.00% 0.00%
Range of risk-free interest rates (in percentage)     0.30% 2.50%
Awards granted in period (shares) 0 0 300,000  
1998 Employee Qualified Stock Purchase Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected dividends (in percentage)     0.00% 0.00%
Awards granted in period (shares) 0 0    
1998 Employee Qualified Stock Purchase Plan | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Range of expected volatilities (in percentage)     39.40% 36.60%
Range of expected lives (in years)     6 months 6 months
Range of risk-free interest rates (in percentage)     0.30% 2.40%
1998 Employee Qualified Stock Purchase Plan | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Range of expected volatilities (in percentage)     45.80% 39.70%
Range of expected lives (in years)     2 years 2 years
Range of risk-free interest rates (in percentage)     0.50% 2.50%