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Investments - Summary of Marketable Securities (Detail) - USD ($)
$ in Thousands
Apr. 30, 2019
Jan. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 1,329,376 $ 1,312,220
Unrealized Gains 970 470
Unrealized Losses (261) (683)
Aggregate Fair Value 1,330,085 1,312,007
U.S. treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 484,846 396,347
Unrealized Gains 214 61
Unrealized Losses (79) (178)
Aggregate Fair Value 484,981 396,230
U.S. agency obligations    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 188,057 241,914
Unrealized Gains 117 73
Unrealized Losses (61) (151)
Aggregate Fair Value 188,113 241,836
Corporate bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 373,946 419,784
Unrealized Gains 637 336
Unrealized Losses (121) (352)
Aggregate Fair Value 374,462 419,768
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 282,527 254,175
Unrealized Gains 2 0
Unrealized Losses 0 (2)
Aggregate Fair Value 282,529 254,173
Included in cash and cash equivalents    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 219,944 216,270
Unrealized Gains 0 0
Unrealized Losses 0 0
Aggregate Fair Value 219,944 216,270
Included in marketable securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 1,109,432 1,095,950
Unrealized Gains 970 470
Unrealized Losses (261) (683)
Aggregate Fair Value $ 1,110,141 $ 1,095,737