Commitments and Contingencies - Narrative (Details) - USD ($)	9 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	May 01, 2019
Lessee, Lease, Description [Line Items]
Discount rate	7.25%
Operating leases, rent expense	$ 717,000
Operating leases, rent expense		$ 583,000
Corporate headquarters
Lessee, Lease, Description [Line Items]
Operating leases, rent expense	71,000
Operating leases, rent expense		68,000
Rockville, MD
Lessee, Lease, Description [Line Items]
Operating leases, rent expense	454,000
Operating leases, rent expense		454,000
Gaithersburg, MD
Lessee, Lease, Description [Line Items]
Operating leases, rent expense	$ 0
Termination period	7 days
Operating leases, rent expense		41,000
Rockville, MD New Location
Lessee, Lease, Description [Line Items]
Operating leases, rent expense	$ 193,000
Operating leases, rent expense		$ 21,000
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Weighted average remaining lease term			7 years 8 months 5 days
Weighted average discount rate			7.25%