Investments - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Apr. 30, 2018	Jan. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 3,126,447	$ 2,554,514
Unrealized Gains	26	16
Unrealized Losses	(3,966)	(3,423)
Aggregate Fair Value	3,122,507	2,551,107
U.S. agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	553,880	683,551
Unrealized Gains	0	0
Unrealized Losses	(1,002)	(1,127)
Aggregate Fair Value	552,878	682,424
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,391,222	797,977
Unrealized Gains	7	0
Unrealized Losses	(1,428)	(1,142)
Aggregate Fair Value	1,389,801	796,835
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	420,352	470,259
Unrealized Gains	19	16
Unrealized Losses	(1,527)	(1,154)
Aggregate Fair Value	418,844	469,121
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	760,993	602,727
Unrealized Gains	0	0
Unrealized Losses	(9)	0
Aggregate Fair Value	760,984	602,727
Included in cash and cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	792,889	417,613
Unrealized Gains	4	0
Unrealized Losses	(9)	(1)
Aggregate Fair Value	792,884	417,612
Included in marketable securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,333,558	2,136,901
Unrealized Gains	22	16
Unrealized Losses	(3,957)	(3,422)
Aggregate Fair Value	$ 2,329,623	$ 2,133,495

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
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