Balance Sheet Components - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 433,508,000	$ 678,393,000
Other-than-temporary impairment loss	$ 0
Percentage of pre-launch inventory capitalized of aggregate inventory balance	37.00%	29.00%
Net unbilled co-development revenue	$ 4,030,000	$ 0
Profit share liability	10,725,000	6,547,000
Astellas Agreement [Member]
Schedule of Available-for-sale Securities [Line Items]
Unbilled contract assets, change in estimated variable consideration	4,000,000.0
AstraZeneca Agreements [Member]
Schedule of Available-for-sale Securities [Line Items]
Net unbilled co-development revenue	3,700,000
AstraZeneca Agreements [Member] \| China [Member]
Schedule of Available-for-sale Securities [Line Items]
Profit share liability	$ 7,000,000.0	7,000,000.0
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Contractual maturities of available-for-sale investments	4 years
Foreign subsidiaries [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 76,500,000	$ 66,000,000.0

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Percentage of pre-launch inventory capitalized of aggregate inventory balance.

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- Definition

Supplies, net.

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Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in estimate of transaction price which increases (decreases) right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time. Includes, but is not limited to, change in assessment of whether estimate of variable consideration is constrained.

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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current.

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The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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