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Investments (Summary of Marketable Securities) (Detail) - USD ($)
$ in Thousands
Jan. 31, 2019
Jan. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 1,312,220 $ 2,554,514
Unrealized Gains 470 16
Unrealized Losses (683) (3,423)
Aggregate Fair Value 1,312,007 2,551,107
U.S. treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 396,347 797,977
Unrealized Gains 61 0
Unrealized Losses (178) (1,142)
Aggregate Fair Value 396,230 796,835
U.S. agency obligations    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 241,914 683,551
Unrealized Gains 73 0
Unrealized Losses (151) (1,127)
Aggregate Fair Value 241,836 682,424
Corporate bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 419,784 470,259
Unrealized Gains 336 16
Unrealized Losses (352) (1,154)
Aggregate Fair Value 419,768 469,121
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 254,175 602,727
Unrealized Gains 0 0
Unrealized Losses (2) 0
Aggregate Fair Value 254,173 602,727
Included in cash and cash equivalents    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 216,270 417,613
Unrealized Gains 0 0
Unrealized Losses 0 (1)
Aggregate Fair Value 216,270 417,612
Included in marketable securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 1,095,950 2,136,901
Unrealized Gains 470 16
Unrealized Losses (683) (3,422)
Aggregate Fair Value $ 1,095,737 $ 2,133,495