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Common Stock and Stockholders' Equity - Stock Options (Detail) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended
Apr. 30, 2018
Jan. 31, 2018
Outstanding Stock Options    
Beginning Balance (in shares) 6,595,486  
Stock options granted (in shares) 0  
Stock options exercised (in shares) (516,354)  
Stock options canceled (in shares) 0  
Ending Balance (in shares) 6,079,132  
Vested and expected to vest (in shares) 6,079,126  
Exercisable (in shares) 6,078,882  
Weighted- Average Exercise Price    
Beginning Balance (in dollars per share) $ 4.23  
Stock option grants (in dollars per share) 0.00  
Stock options exercised (in dollars per share) 5.03  
Stock options canceled (in dollars per share) 0.00  
Ending Balance (in dollars per share) 4.16  
Vested and expected to vest, Weighted-Average Exercise Price (in dollars per share) 4.16  
Exercisable, Weighted-Average Exercise Price (in dollars per share) $ 4.16  
Aggregate Intrinsic Value (in millions) $ 734 $ 763
Vested and expected to vest, Aggregate Intrinsic Value 734  
Exercisable, Aggregate Intrinsic Value $ 734