Significant Accounting Policies - Estimated Fair Value of Assets and Liability (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Asset:
Asset, fair value	$ 374,911	$ 473,882
Level 1
Asset:
Asset, fair value	118,282	136,622
Level 2
Asset:
Asset, fair value	256,629	337,260
Money Market Fund
Asset:
Asset, fair value	95,063	88,266
Money Market Fund | Level 1
Asset:
Asset, fair value	95,063	88,266
U.S. Treasury Shares
Asset:
Asset, fair value	23,219	48,356
U.S. Treasury Shares | Level 1
Asset:
Asset, fair value	23,219	48,356
Corporate Bonds
Asset:
Asset, fair value	83,587	179,864
Corporate Bonds | Level 2
Asset:
Asset, fair value	83,587	179,864
Commercial Paper
Asset:
Asset, fair value	121,773	106,650
Commercial Paper | Level 2
Asset:
Asset, fair value	121,773	106,650
Agency Securities
Asset:
Asset, fair value		20,782
Agency Securities | Level 2
Asset:
Asset, fair value		20,782
Repo Securities
Asset:
Asset, fair value	8,000	10,000
Repo Securities | Level 2
Asset:
Asset, fair value	8,000	10,000
Asset Backed Securities
Asset:
Asset, fair value	8,816	8,599
Asset Backed Securities | Level 2
Asset:
Asset, fair value	8,816	8,599
Certificate of Deposit
Asset:
Asset, fair value	21,553	6,125
Certificate of Deposit | Level 2
Asset:
Asset, fair value	21,553	6,125
Non-US Government
Asset:
Asset, fair value	12,900	5,240
Non-US Government | Level 2
Asset:
Asset, fair value	$ 12,900	$ 5,240