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Stock-based Compensation Expense (Assumption Used to Estimate the Fair Value of Stock-Based Awards) (Details) - shares
shares in Millions
3 Months Ended
Apr. 30, 2020
Apr. 30, 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) 0.3  
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%
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%