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Common Stock and Stockholders' Equity (Summary of Information Related to Restricted Stock Units Activity) (Detail) - Restricted Stock Units (RSU) - $ / shares
12 Months Ended
Jan. 31, 2019
Jan. 31, 2018
Jan. 31, 2017
Restricted Stock Units      
Beginning Balance, Number of Shares 12,819,516    
Restricted stock units granted, Number of Shares 6,991,459    
Restricted stock units vested, Number of Shares (5,893,133)    
Restricted stock units forfeited, Number of Shares (904,553)    
Ending Balance, Number of Shares 13,013,289 12,819,516  
Weighted-Average Grant Date Fair Value      
Beginning Balance (in usd per share) $ 84.77    
Restricted stock units granted (in usd per share) 129.62 $ 88.90 $ 75.71
Restricted stock units vested (in usd per share) 84.58    
Restricted stock units forfeited (in usd per share) 96.79    
Ending Balance (in usd per share) $ 108.12 $ 84.77