Fair Value Measurement (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Level 2
Assets:
Available-for-sale investments	$ 896,862	$ 1,003,348
Recurring
Assets:
Total assets	1,035,151	1,182,650
Liabilities:
Development derivative liability	11,607	0
Recurring \| Corporate notes and bonds
Assets:
Available-for-sale investments	403,551	687,469
Recurring \| Corporate commercial paper
Assets:
Available-for-sale investments	487,430	313,497
Recurring \| Obligations of U.S. government agencies
Assets:
Available-for-sale investments	5,881	2,382
Recurring \| Money market funds
Assets:
Money market funds	138,289	179,302
Recurring \| Level 1
Assets:
Total assets	138,289	179,302
Liabilities:
Development derivative liability	0	0
Recurring \| Level 1 \| Corporate notes and bonds
Assets:
Available-for-sale investments	0	0
Recurring \| Level 1 \| Corporate commercial paper
Assets:
Available-for-sale investments	0	0
Recurring \| Level 1 \| Obligations of U.S. government agencies
Assets:
Available-for-sale investments	0	0
Recurring \| Level 1 \| Money market funds
Assets:
Money market funds	138,289	179,302
Recurring \| Level 2
Assets:
Total assets	896,862	1,003,348
Liabilities:
Development derivative liability	0	0
Recurring \| Level 2 \| Corporate notes and bonds
Assets:
Available-for-sale investments	403,551	687,469
Recurring \| Level 2 \| Corporate commercial paper
Assets:
Available-for-sale investments	487,430	313,497
Recurring \| Level 2 \| Obligations of U.S. government agencies
Assets:
Available-for-sale investments	5,881	2,382
Recurring \| Level 2 \| Money market funds
Assets:
Money market funds	0	0
Recurring \| Level 3
Assets:
Total assets	0	0
Liabilities:
Development derivative liability	11,607	0
Recurring \| Level 3 \| Corporate notes and bonds
Assets:
Available-for-sale investments	0	0
Recurring \| Level 3 \| Corporate commercial paper
Assets:
Available-for-sale investments	0	0
Recurring \| Level 3 \| Obligations of U.S. government agencies
Assets:
Available-for-sale investments	0	0
Recurring \| Level 3 \| Money market funds
Assets:
Money market funds	$ 0	$ 0

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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