Organization and Summary of Significant Accounting Policies - Narrative (Details)	6 Months Ended
Sep. 30, 2021 USD ($) segment shares	Oct. 26, 2021 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of operating segments \| segment	1
Number of reporting segments \| segment	1
Cash, cash equivalents and marketable securities	$ 616,000,000
Sumitomo Dainippon Pharma Co., Ltd. \| Majority Shareholder
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of shares owned (in shares) \| shares	50,041,181
Ownership percentage	53.80%
Sumitomo Dainippon Pharma Co., Ltd. \| Letter Agreement with Sumitomo Dainippon Pharma, Co., Ltd. \| Term Loan \| Majority Shareholder
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Available borrowing capacity	$ 41,300,000
Pfizer \| Subsequent Event
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Outstanding payment from milestones achieved		$ 3,600,000,000
Richter \| Subsequent Event
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Outstanding payment from milestones achieved		$ 122,500,000

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Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

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-Publisher FASB
-Name Accounting Standards Codification
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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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