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Summary of Significant Accounting Policies - Schedule of Significant Balance of Accounts Receivable (Detail) - Accounts Receivable [Member] - Credit Concentration Risk [Member] | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Astellas Agreement [Member] |  |
Accounting Policy [Line Items] |  |
Concentration risk, percentage | 17.00% | 74.00%
AstraZeneca Agreements [Member] |  |
Accounting Policy [Line Items] |  |
Concentration risk, percentage | 81.00% | 26.00%

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