Stockholders' Equity - Summary of Stock Option Activity (Detail) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Oct. 31, 2018	Oct. 31, 2018	Jan. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of shares, Options outstanding, Beginning Balance		16,024,146
Number of shares, Options granted	0	175,000
Number of shares, Options exercised		(2,012,604)
Number of shares, Options forfeited/cancelled		(254,019)
Number of shares, Options outstanding, Ending Balance	13,932,523	13,932,523	16,024,146
Number of shares, Options vested and exercisable	6,531,474	6,531,474
Number of shares, Options vested and exercisable and expected to vest thereafter	13,932,523	13,932,523
Weighted average exercise price, Options outstanding, Beginning Balance		$ 16.76
Weighted average exercise price, Options granted		77.88
Weighted average exercise price, Options exercised		9.58
Weighted average exercise price, Options forfeited/cancelled		7.59
Weighted average exercise price, Options outstanding, Ending Balance	$ 18.74	18.74	$ 16.76
Weighted average exercise price, Options vested and exercisable	5.20	5.20
Weighted average exercise price, Options vested and exercisable and expected to vest thereafter	$ 18.74	$ 18.74
Weighted average remaining contractual term (in years), Options outstanding		5 years 6 months	6 years 1 month 6 days
Weighted average remaining contractual term (in years), Options vested and exercisable		4 years 2 months 12 days
Weighted average remaining contractual term (in years), Options vested and exercisable and expected to vest thereafter		5 years 6 months
Aggregate intrinsic value, Options outstanding	$ 1,011,683,656	$ 1,011,683,656	$ 738,648,507
Aggregate intrinsic value, Options vested and exercisable	562,702,409	562,702,409
Aggregate intrinsic value, Options vested and exercisable and expected to vest thereafter	$ 1,011,683,656	$ 1,011,683,656