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Stock-based Compensation Expense (Assumption Used to Estimate the Fair Value of Stock-Based Awards) (Details)
3 Months Ended 9 Months Ended
Oct. 31, 2018
Oct. 31, 2017
Oct. 31, 2018
Oct. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected dividends     0.00%  
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%  
Performance Stock Unit | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Range of risk-free interest rates       1.00%
Performance Stock Unit | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Range of risk-free interest rates       1.20%
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% 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 32.70% 31.70% 32.70% 31.40%
Range of expected lives 6 months 6 months 6 months 6 months
Range of risk-free interest rates 2.30% 1.20% 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 34.60% 33.40% 37.50% 33.70%
Range of expected lives 2 years 2 years 2 years 2 years
Range of risk-free interest rates 2.80% 1.40% 2.80% 1.40%