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Fair Value Measurements - Fair Value Hierarchy for Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Oct. 31, 2018
Jan. 31, 2018
Assets    
Short-term investments $ 584,283 $ 441,779 [1]
Commercial paper [Member]    
Assets    
Short-term investments 4,969 19,914
Corporate notes and bonds [Member]    
Assets    
Short-term investments 172,087 159,741
U.S. treasury securities [Member]    
Assets    
Short-term investments 272,693 110,576
Certificates of deposits [Member]    
Assets    
Short-term investments 9,547  
Asset-backed securities [Member]    
Assets    
Short-term investments 80,164 67,451
Foreign government bonds [Member]    
Assets    
Short-term investments 2,981 1,486
Mortgage backed securities [Member]    
Assets    
Short-term investments 6,119 11,480
U.S. agency obligations [Member]    
Assets    
Short-term investments 35,723 71,131
Fair value, measurements recurring [Member]    
Assets    
Total 616,348 479,805
Liabilities    
Total 117 391
Fair value, measurements recurring [Member] | Money market funds [Member]    
Assets    
Cash equivalents 6,868 25,820
Fair value, measurements recurring [Member] | Commercial paper [Member]    
Assets    
Cash equivalents 1,846 1,999
Short-term investments 4,968 19,914
Fair value, measurements recurring [Member] | Corporate notes and bonds [Member]    
Assets    
Cash equivalents 1,045 2,080
Short-term investments 172,087 159,741
Fair value, measurements recurring [Member] | U.S. treasury securities [Member]    
Assets    
Cash equivalents 22,271 8,000
Short-term investments 272,694 110,576
Fair value, measurements recurring [Member] | Certificates of deposits [Member]    
Assets    
Short-term investments 9,547  
Fair value, measurements recurring [Member] | Asset-backed securities [Member]    
Assets    
Short-term investments 80,164 67,451
Fair value, measurements recurring [Member] | Foreign government bonds [Member]    
Assets    
Short-term investments 2,981 1,486
Fair value, measurements recurring [Member] | Mortgage backed securities [Member]    
Assets    
Short-term investments 6,119 11,480
Fair value, measurements recurring [Member] | U.S. agency obligations [Member]    
Assets    
Short-term investments 35,723 71,131
Fair value, measurements recurring [Member] | Foreign currency derivative contracts [Member]    
Assets    
Foreign currency derivative contracts 35 127
Liabilities    
Foreign currency derivative contracts 117 391
Fair value, measurements recurring [Member] | Level 1 [Member]    
Assets    
Total 6,868 25,820
Fair value, measurements recurring [Member] | Level 1 [Member] | Money market funds [Member]    
Assets    
Cash equivalents 6,868 25,820
Fair value, measurements recurring [Member] | Level 2 [Member]    
Assets    
Total 609,480 453,985
Liabilities    
Total 117 391
Fair value, measurements recurring [Member] | Level 2 [Member] | Commercial paper [Member]    
Assets    
Cash equivalents 1,846 1,999
Short-term investments 4,968 19,914
Fair value, measurements recurring [Member] | Level 2 [Member] | Corporate notes and bonds [Member]    
Assets    
Cash equivalents 1,045 2,080
Short-term investments 172,087 159,741
Fair value, measurements recurring [Member] | Level 2 [Member] | U.S. treasury securities [Member]    
Assets    
Cash equivalents 22,271 8,000
Short-term investments 272,694 110,576
Fair value, measurements recurring [Member] | Level 2 [Member] | Certificates of deposits [Member]    
Assets    
Short-term investments 9,547  
Fair value, measurements recurring [Member] | Level 2 [Member] | Asset-backed securities [Member]    
Assets    
Short-term investments 80,164 67,451
Fair value, measurements recurring [Member] | Level 2 [Member] | Foreign government bonds [Member]    
Assets    
Short-term investments 2,981 1,486
Fair value, measurements recurring [Member] | Level 2 [Member] | Mortgage backed securities [Member]    
Assets    
Short-term investments 6,119 11,480
Fair value, measurements recurring [Member] | Level 2 [Member] | U.S. agency obligations [Member]    
Assets    
Short-term investments 35,723 71,131
Fair value, measurements recurring [Member] | Level 2 [Member] | Foreign currency derivative contracts [Member]    
Assets    
Foreign currency derivative contracts 35 127
Liabilities    
Foreign currency derivative contracts $ 117 $ 391
[1] See note 1 of the notes to the condensed consolidated financial statements for a summary of adjustments.