Commitments and Contingencies - Narrative (Details)	12 Months Ended
	Apr. 30, 2020 USD ($)
Minimum
Commitments and Contingencies [Line Items]
Royalty fee per tumor sample	$ 0
Royalty payment, as percent of contract price	2.00%
Maximum
Commitments and Contingencies [Line Items]
Royalty fee per tumor sample	$ 7,000
Royalty payment, as percent of contract price	5.00%