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Short-Term Investments - Schedule of Short-Term Investments (Detail) - USD ($)
$ in Thousands
Apr. 30, 2018
Jan. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost $ 460,070 $ 443,272
Available-for-sale securities, Gross unrealized gains 34 7
Available-for-sale securities, Gross unrealized losses (2,035) (1,500)
Available-for-sale securities, Estimated fair value 458,069 441,779 [1]
Certificates of deposits [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 11,030  
Available-for-sale securities, Gross unrealized gains 4  
Available-for-sale securities, Estimated fair value 11,034  
Asset-backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 70,068 67,875
Available-for-sale securities, Gross unrealized gains 1  
Available-for-sale securities, Gross unrealized losses (615) (424)
Available-for-sale securities, Estimated fair value 69,454 67,451
Commercial paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 21,286 19,926
Available-for-sale securities, Gross unrealized gains 1  
Available-for-sale securities, Gross unrealized losses (9) (12)
Available-for-sale securities, Estimated fair value 21,278 19,914
Corporate notes and bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 155,457 160,499
Available-for-sale securities, Gross unrealized gains 24 1
Available-for-sale securities, Gross unrealized losses (1,011) (759)
Available-for-sale securities, Estimated fair value 154,470 159,741
Foreign government bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 1,503 1,504
Available-for-sale securities, Gross unrealized losses (23) (18)
Available-for-sale securities, Estimated fair value 1,480 1,486
Mortgage backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 13,027 11,555
Available-for-sale securities, Gross unrealized gains 2  
Available-for-sale securities, Gross unrealized losses (100) (75)
Available-for-sale securities, Estimated fair value 12,929 11,480
U.S. agency obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 64,765 71,206
Available-for-sale securities, Gross unrealized gains 1 1
Available-for-sale securities, Gross unrealized losses (59) (76)
Available-for-sale securities, Estimated fair value 64,707 71,131
U.S. treasury securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 122,934 110,707
Available-for-sale securities, Gross unrealized gains 1 5
Available-for-sale securities, Gross unrealized losses (218) (136)
Available-for-sale securities, Estimated fair value $ 122,717 $ 110,576
[1] See note 1 of the notes to the condensed consolidated financial statements for a summary of adjustments.