Significant Accounting Policies - Estimated Fair Value of Assets and Liability (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Asset:
Asset, fair value	$ 419,883	$ 473,882
Money Market Fund
Asset:
Asset, fair value	176,345	88,266
U.S. Treasury Shares
Asset:
Asset, fair value	22,986	48,356
Corporate Bonds
Asset:
Asset, fair value	87,225	179,864
Commercial Paper
Asset:
Asset, fair value	100,460	106,650
Agency Securities
Asset:
Asset, fair value	15,772	20,782
Repo Securities
Asset:
Asset, fair value	9,000	10,000
Asset Backed Securities
Asset:
Asset, fair value	2,867	8,599
Certificates of Deposit
Asset:
Asset, fair value		6,125
Non-US Government
Asset:
Asset, fair value	5,228	5,240
Level 1
Asset:
Asset, fair value	199,331	136,622
Level 1 | Money Market Fund
Asset:
Asset, fair value	176,345	88,266
Level 1 | U.S. Treasury Shares
Asset:
Asset, fair value	22,986	48,356
Level 2
Asset:
Asset, fair value	220,552	337,260
Level 2 | Corporate Bonds
Asset:
Asset, fair value	87,225	179,864
Level 2 | Commercial Paper
Asset:
Asset, fair value	100,460	106,650
Level 2 | Agency Securities
Asset:
Asset, fair value	15,772	20,782
Level 2 | Repo Securities
Asset:
Asset, fair value	9,000	10,000
Level 2 | Asset Backed Securities
Asset:
Asset, fair value	2,867	8,599
Level 2 | Certificates of Deposit
Asset:
Asset, fair value		6,125
Level 2 | Non-US Government
Asset:
Asset, fair value	$ 5,228	$ 5,240