License and Collaboration Agreements (Details) - USD ($)				1 Months Ended		12 Months Ended
	Apr. 30, 2019	Dec. 07, 2018	Jun. 05, 2018	Dec. 31, 2018	Jul. 31, 2018	Mar. 31, 2020	Mar. 31, 2019	Oct. 31, 2019	Feb. 28, 2019
License Agreement [Line Items]
Payments for license agreement	$ 13,000,000
Capitalized costs, current						$ 6,951,000	$ 13,416,000
Oxford BioMedica (UK) Ltd.
License Agreement [Line Items]
Payments for license agreement			$ 30,000,000			$ 15,200,000	31,500,000
Payments for license agreement, developmental milestones			55,000,000
Payments for license agreement, regulatory and sales milestones			$ 757,500,000
License agreement term			10 years
License agreement, termination notice prior term						2 months
License agreement costs						$ 9,200,000	5,300,000
Benitec Biopharma Limited
License Agreement [Line Items]
Payments for license agreement						2,800,000	12,100,000
The University of Massachusetts Medical School
License Agreement [Line Items]
Payments for license agreement				$ 10,000,000		3,000,000	10,200,000
License agreement term		10 years
License agreement costs						$ 8,800,000	1,200,000
Payments for license and collaboration agreement, development and regulatory milestones, total		$ 24,500,000.0
Payments for license and collaboration agreement, commercial milestones, total		$ 39,800,000.0
Research and development arrangement								$ 1,000,000	$ 1,000,000
Termination notice term, uncured material breach		90 days				90 days
Termination notice term		60 days				60 days
Research and development | Oxford BioMedica (UK) Ltd.
License Agreement [Line Items]
Payments for license agreement			$ 25,000,000
Research and development | Benitec Biopharma Limited
License Agreement [Line Items]
License agreement costs					$ 10,000,000
Other than research and development expenses | Benitec Biopharma Limited
License Agreement [Line Items]
License agreement costs						$ 1,800,000	$ 4,600,000
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
Prepaid Expenses and Other Current Assets | Oxford BioMedica (UK) Ltd.
License Agreement [Line Items]
Capitalized costs, current						$ 1,700,000
Minimum | Oxford BioMedica (UK) Ltd.
License Agreement [Line Items]
Royalty percentage			7.00%
Fee if developmental milestones are not me			$ 500,000
Maximum | Oxford BioMedica (UK) Ltd.
License Agreement [Line Items]
Royalty percentage			10.00%
Fee if developmental milestones are not me			$ 1,000,000