Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands		3 Months Ended
Aug. 03, 2020	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)
Accounting Policies [Abstract]
Number of operating segments \| segment		1
Number of reporting segments \| segment		1
Net loss \| $		$ 32,860	$ 67,904
Cash and cash equivalents used in operations \| $		$ 61,984	$ 66,811
Sumitomo Dainippon Pharma Co., Ltd. \| New Letter Agreement with Sumitomo Dainippon Pharma, Co., Ltd. \| Term Loan \| Majority Shareholder \| Subsequent Event
Debt Instrument [Line Items]
Facility term	5 years

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