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Stock-based Compensation Expense Assumption Used to Estimate the Fair Value of Stock-Based Awards (Details)
6 Months Ended
Jul. 31, 2015
Jul. 31, 2014
Performance Stock Unit    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Range of expected volatilities 27.00% 30.00%
Expected dividends 0.00% 0.00%
Range of risk-free interest rates 0.20% 0.10%
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 27.70% 30.00%
Range of expected lives (in years) 6 months 6 months
Range of risk-free interest rates 0.10% 0.10%
Employee Qualified Stock Purchase Plan 1998 ESP Plan | Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Range of expected volatilities 28.20% 33.00%
Range of expected lives (in years) 2 years 2 years
Range of risk-free interest rates 0.60% 0.40%