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Investments - Summary of Marketable Securities (Detail) - USD ($)
$ in Thousands
Oct. 31, 2018
Jan. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 1,250,422 $ 2,554,514
Unrealized Gains 7 16
Unrealized Losses (1,839) (3,423)
Aggregate Fair Value 1,248,590 2,551,107
U.S. agency obligations    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 188,241 683,551
Unrealized Gains 1 0
Unrealized Losses (375) (1,127)
Aggregate Fair Value 187,867 682,424
U.S. treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 376,887 797,977
Unrealized Gains 0 0
Unrealized Losses (437) (1,142)
Aggregate Fair Value 376,450 796,835
Corporate bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 402,034 470,259
Unrealized Gains 6 16
Unrealized Losses (1,027) (1,154)
Aggregate Fair Value 401,013 469,121
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 283,260 602,727
Unrealized Gains 0 0
Unrealized Losses 0 0
Aggregate Fair Value 283,260 602,727
Included in cash and cash equivalents    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 239,214 417,613
Unrealized Gains 0 0
Unrealized Losses 0 (1)
Aggregate Fair Value 239,214 417,612
Included in marketable securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 1,011,208 2,136,901
Unrealized Gains 7 16
Unrealized Losses (1,839) (3,422)
Aggregate Fair Value $ 1,009,376 $ 2,133,495