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Stock-based Compensation Expense (Assumption Used to Estimate the Fair Value of Stock-Based Awards) (Details)
3 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Performance Stock Unit    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Range of expected volatilities 35.70% 31.80%
Expected dividends 0.00% 0.00%
Range of risk-free interest rates 2.00% 1.00%
Employee Qualified Stock Purchase Plan 1998 ESP Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected dividends 0.00% 0.00%
Employee Qualified Stock Purchase Plan 1998 ESP Plan | Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Range of expected volatilities 33.50% 31.40%
Range of expected lives 5 months 5 months
Range of risk-free interest rates 1.90% 0.90%
Employee Qualified Stock Purchase Plan 1998 ESP Plan | Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Range of expected volatilities 37.50% 33.70%
Range of expected lives 2 years 2 years
Range of risk-free interest rates 2.30% 1.30%