License and Collaboration Agreements License and Collaboration Agreements (Details) - USD ($)		1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 05, 2018	Apr. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2020	Oct. 31, 2019
License Agreement [Line Items]
Capitalized costs, current			$ 5,469,000		$ 5,469,000		$ 6,951,000
Oxford BioMedica (UK) Ltd.
License Agreement [Line Items]
Payments for license agreement			600,000	$ 5,200,000	1,100,000	$ 6,500,000
Payments for license and collaboration agreement, development and regulatory milestones, achieved during period		$ 13,000,000.0
License agreement costs			1,700,000	2,700,000	3,400,000	4,100,000
Benitec Biopharma Limited
License Agreement [Line Items]
Payments for license agreement				1,700,000		2,400,000
License agreement costs				400,000		2,100,000
The University of Massachusetts Medical School
License Agreement [Line Items]
Payments for license agreement			12,000	800,000	12,000	1,800,000
License agreement costs			400,000	$ 1,700,000	800,000	$ 2,700,000
Additional payment due to UMMS								$ 1,000,000.0
Prepaid Expenses and Other Current Assets and Other Noncurrent Assets | Oxford BioMedica (UK) Ltd.
License Agreement [Line Items]
Payments for license agreement	$ 5,000,000.0
Prepaid Expenses and Other Current Assets | Oxford BioMedica (UK) Ltd.
License Agreement [Line Items]
Capitalized costs, current			$ 1,300,000		$ 1,300,000