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Stock-based Compensation Expense Assumption Used to Estimate the Fair Value of Stock-Based Awards (Details)
3 Months Ended 6 Months Ended
Jul. 31, 2017
Jul. 31, 2017
Jul. 31, 2016
Performance Stock Unit      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Range of expected volatilities 31.80% 31.80%  
Expected dividends 0.00% 0.00% 0.00%
Range of risk-free interest rates 1.20%    
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%
Employee Qualified Stock Purchase Plan 1998 ESP Plan | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Range of expected volatilities   31.40% 35.00%
Range of expected lives (in years)   6 months 6 months
Range of risk-free interest rates   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   33.70% 40.20%
Range of expected lives (in years)   2 years 2 years
Range of risk-free interest rates   1.30% 0.90%