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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, 2017
Oct. 31, 2016
Oct. 31, 2017
Oct. 31, 2016
Performance Stock Unit        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Range of expected volatilities     31.80%  
Expected dividends     0.00% 0.00%
Performance Stock Unit | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Range of expected volatilities       38.40%
Range of risk-free interest rates     1.00% 0.60%
Performance Stock Unit | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Range of expected volatilities       38.60%
Range of risk-free interest rates     1.20% 0.70%
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 31.70% 31.00% 31.40% 30.00%
Range of expected lives (in years) 6 months 6 months 6 months 6 months
Range of risk-free interest rates 1.20% 0.50% 0.90% 0.50%
Employee Qualified Stock Purchase Plan 1998 ESP Plan | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Range of expected volatilities 33.40% 33.90% 33.70% 40.20%
Range of expected lives (in years) 2 years 2 years 2 years 2 years
Range of risk-free interest rates 1.40% 0.80% 1.40% 0.90%