Grant Income - Additional Information (Detail)	1 Months Ended	3 Months Ended								12 Months Ended
	May 31, 2016 USD ($)	Apr. 30, 2017 USD ($)	Jan. 31, 2017 USD ($)	Oct. 31, 2016 USD ($)	Jul. 31, 2016 USD ($)	Apr. 30, 2016 USD ($)	Jan. 31, 2016 USD ($)	Oct. 31, 2015 USD ($)	Jul. 31, 2015 USD ($)	Apr. 30, 2017 USD ($) Agreement	Apr. 30, 2016 USD ($)	Apr. 30, 2015 USD ($)
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Number of grant income agreements | Agreement										2
Revenue recognized		$ 114,000	$ 248,000	$ 197,000	$ 975,000	$ 314,000	$ 348,000	$ 667,000	$ 839,000	$ 1,504,000	$ 2,133,000	$ 1,804,000
Grant income received or due to be received under agreement		297,000				$ 356,000				297,000	356,000
Technology Strategy Board (TSB) [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenue recognized										1,200,000	1,800,000	1,500,000
Total amount over lifetime of agreement		7,300,000								7,300,000
Grant income received or due to be received under agreement		$ 5,900,000								5,900,000
JDRF [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenue recognized through achievement of multiple milestones	$ 0									$ 200,000	$ 300,000	$ 300,000