Significant Accounting Policies - Estimated Fair Value of Assets and Liability (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Asset:
Asset, fair value	$ 473,882
Level 1
Asset:
Asset, fair value	136,622
Level 2
Asset:
Asset, fair value	337,260
Money Market Fund
Asset:
Asset, fair value	88,266	$ 10,078
Money Market Fund | Level 1
Asset:
Asset, fair value	88,266	$ 10,078
U.S. Treasury Shares
Asset:
Asset, fair value	48,356
U.S. Treasury Shares | Level 1
Asset:
Asset, fair value	48,356
Corporate Bonds
Asset:
Asset, fair value	179,864
Corporate Bonds | Level 2
Asset:
Asset, fair value	179,864
Commercial Paper
Asset:
Asset, fair value	106,650
Commercial Paper | Level 2
Asset:
Asset, fair value	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	10,000
Repo Securities | Level 2
Asset:
Asset, fair value	10,000
Asset Backed Securities
Asset:
Asset, fair value	8,599
Asset Backed Securities | Level 2
Asset:
Asset, fair value	8,599
Certificate of Deposit
Asset:
Asset, fair value	6,125
Certificate of Deposit | Level 2
Asset:
Asset, fair value	6,125
Non-US Government
Asset:
Asset, fair value	5,240
Non-US Government | Level 2
Asset:
Asset, fair value	$ 5,240