Stockholders' Equity - Summary of Stock Option Activity (Detail)	6 Months Ended	12 Months Ended
	Jul. 31, 2019 USD ($) $ / shares shares	Jan. 31, 2019 USD ($) $ / shares shares
Number of shares
Options outstanding, Beginning balance (in shares) | shares	12,961,397
Options granted (in shares) | shares	1,355,345
Options exercised (in shares) | shares	(1,014,592)
Options forfeited/cancelled (in shares) | shares	(196,824)
Options outstanding, Ending balance (in shares) | shares	13,105,326	12,961,397
Options vested and exercisable (in shares) | shares	6,478,882
Options vested and exercisable and expected to vest thereafter (in shares) | shares	13,105,326
Weighted average exercise price
Options outstanding, Beginning balance (in usd per share) | $ / shares	$ 19.43
Options granted (in usd per share) | $ / shares	134.98
Options exercised (in usd per share) | $ / shares	6.81
Options forfeited/cancelled (in usd per share) | $ / shares	53.45
Options outstanding, Ending balance (in usd per share) | $ / shares	31.85	$ 19.43
Options vested and exercisable (in usd per share) | $ / shares	5.83
Options vested and exercisable and expected to vest thereafter (in usd per share) | $ / shares	$ 31.85
Weighted average remaining contractual term (in years), Options outstanding	5 years 4 months 24 days	5 years 4 months 24 days
Weighted average remaining contractual term (in years), Options vested and exercisable	3 years 7 months 6 days
Weighted average remaining contractual term (in years), Options vested and exercisable and expected to vest thereafter	5 years 4 months 24 days
Aggregate intrinsic value
Options outstanding | $	$ 1,756,820,772	$ 1,161,695,032
Options vested and exercisable | $	1,037,073,153
Options vested and exercisable and expected to vest thereafter | $	$ 1,756,820,772