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Short-Term Investments - Schedule of Short-Term Investments (Detail) - USD ($)
$ in Thousands
Jul. 31, 2016
Jan. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost $ 269,010 $ 214,113
Available-for-sale securities, Gross Unrealized Gains 216 44
Available-for-sale securities, Gross Unrealized Losses (36) (133)
Available-for-sale securities, Estimated Fair Value 269,190 214,024
Asset-backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 16,660 5,456
Available-for-sale securities, Gross Unrealized Gains 35  
Available-for-sale securities, Gross Unrealized Losses   (2)
Available-for-sale securities, Estimated Fair Value 16,695 5,454
Commercial paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 16,062 5,970
Available-for-sale securities, Estimated Fair Value 16,062 5,970
Corporate notes and bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 74,196 38,341
Available-for-sale securities, Gross Unrealized Gains 72 26
Available-for-sale securities, Gross Unrealized Losses (23) (40)
Available-for-sale securities, Estimated Fair Value 74,245 38,327
U.S. agency obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 101,803 124,626
Available-for-sale securities, Gross Unrealized Gains 64 14
Available-for-sale securities, Gross Unrealized Losses (12) (54)
Available-for-sale securities, Estimated Fair Value 101,855 124,586
U.S. treasury securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 60,289 39,720
Available-for-sale securities, Gross Unrealized Gains 45 4
Available-for-sale securities, Gross Unrealized Losses (1) (37)
Available-for-sale securities, Estimated Fair Value $ 60,333 $ 39,687