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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, 2015
Oct. 31, 2014
Oct. 31, 2015
Oct. 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]        
Range of expected volatilities 29.00%      
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   29.00% 28.00% 29.00%
Range of expected lives (in years) 6 months 6 months 6 months 6 months
Range of risk-free interest rates 0.20% 0.00% 0.10% 0.00%
Employee Qualified Stock Purchase Plan 1998 ESP Plan | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Range of expected volatilities   30.00% 29.00% 33.00%
Range of expected lives (in years) 2 years 2 years 2 years 2 years
Range of risk-free interest rates 0.70% 0.60% 0.70% 0.60%