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Stock-based Compensation Expense (Additional Information) (Details)
$ in Millions
3 Months Ended
Apr. 30, 2019
USD ($)
period
shares
Apr. 30, 2018
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Compensation cost related to non-vested stock options $ 58.5  
Market capitalization 2,000.0  
Allocated share-based compensation expense $ 75.2 $ 54.4
Expected dividend yield 0.00%  
Stock Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Compensation cost related to non-vested stock options, weighted average period of recognition 2 years 7 months 6 days  
Restricted Stock Units (RSUs) and Performance Shares    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Awards vested in period, fair value $ 50.3 96.2
Awards granted in period (shares) | shares 904,800  
Restricted Stock Units (RSUs)    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Awards granted in period (shares) | shares 700,000  
Allocated share-based compensation expense $ 54.4 41.7
Performance Stock Unit    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Awards granted in period (shares) | shares 300,000  
Allocated share-based compensation expense $ 6.5 $ 6.5
Award vesting period 3 years  
Expected dividend yield 0.00% 0.00%
1998 Employee Qualified Stock Purchase Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee stock purchase plan maximum percentage of compensation to purchase shares by eligible participants 15.00%  
Employee stock purchase plan minimum percentage of common stock fair value defined to purchase shares by eligible participants 85.00%  
Employee stock purchase plan, number of exercise periods | period 4  
Employee stock purchase plan, term of exercise period 6 months  
Employee stock purchase plan, term of offering period 24 months  
Expected dividend yield 0.00% 0.00%
Share-based Compensation Award, Tranche One | Performance Stock Unit    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
PSU annual vesting percentage 33.33%  
Share-based Compensation Award, Tranche Two | Performance Stock Unit    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
PSU annual vesting percentage 33.33%  
Share-based Compensation Award, Tranche Three | Performance Stock Unit    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
PSU annual vesting percentage 33.33%