Share-Based Payments - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Jan. 31, 2019	Apr. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Outstanding, Beginning Balance	2,705,845
Shares, Granted	206,790
Shares, Exercised	(357,242)
Shares, Forfeited	(8,375)
Shares, Canceled	(40,183)
Shares, Expired	(16,667)
Shares, Outstanding, Ending Balance	2,490,168	2,705,845
Shares, Vested and expected to vest	2,490,168
Shares, Exercisable	2,135,088
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted Average Exercise Price, Outstanding, Beginning Balance (in usd per share)	$ 2.85
Weighted Average Exercise Price, Granted (in usd per share)	9.86
Weighted Average Exercise Price, Exercised (in usd per share)	2.18
Weighted Average Exercise Price, Forfeited (in usd per share)	0.00
Weighted Average Exercise Price, Canceled (in usd per share)	2.10
Weighted Average Exercise Price, Expired (in usd per share)	0.00
Weighted Average Exercise Price, Outstanding, Ending Balance (in usd per share)	3.45	$ 2.85
Weighted Average Exercise Price, Vested and expected to vest (in usd per share)	3.45
Weighted Average Exercise Price, Exercisable (in usd per share)	$ 2.89
Weighted Average Remaining Contractual Term, Outstanding, Beginning Balance (in years)		5 years 10 months 25 days
Weighted Average Remaining Contractual Life, Granted (in years)	8 years 1 month 18 days
Weighted Average Remaining Contractual Life, Exercised (in years)	4 years 8 months 18 days
Weighted Average Remaining Contractual Term, Outstanding, Ending Balance (in years)	5 years 6 months 25 days
Weighted Average Remaining Contractual Life, Vested and expected to vest (in years)	5 years 6 months 25 days
Weighted Average Remaining Contractual Life, Exercisable (in years)	5 years 1 month 24 days
Aggregate Intrinsic Value, Outstanding, Beginning Balance	$ 5,265
Aggregate Intrinsic Value, Granted	0
Aggregate Intrinsic Value, Exercised	$ 2,782
Aggregate Intrinsic Value, Canceled	$ 370,626
Aggregate Intrinsic Value, Outstanding, Ending Balance	$ 22,036	$ 5,265
Aggregate Intrinsic Value, Vested and expected to vest	22,036
Aggregate Intrinsic Value, Exercisable	$ 20,087
Non-employees
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Outstanding, Beginning Balance	50,000
Shares, Granted	0
Shares, Exercised	0
Shares, Forfeited	0
Shares, Canceled	0
Shares, Expired	0
Shares, Outstanding, Ending Balance	50,000	50,000
Shares, Vested and expected to vest	50,000
Shares, Exercisable	25,836
Directors and employees
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Outstanding, Beginning Balance	2,655,845
Shares, Granted	206,790
Shares, Exercised	(357,242)
Shares, Forfeited	(8,375)
Shares, Canceled	(40,183)
Shares, Expired	(16,667)
Shares, Outstanding, Ending Balance	2,440,168	2,655,845
Shares, Vested and expected to vest	2,440,168
Shares, Exercisable	2,109,252