Significant Accounting Policies - Assets and Liability Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Asset:
Asset, fair value	$ 529,539
Money Market Fund
Asset:
Asset, fair value	156,655	$ 10,078
U.S. Treasury Shares
Asset:
Asset, fair value	48,219
Corporate Bonds
Asset:
Asset, fair value	147,472
Commercial Paper
Asset:
Asset, fair value	135,173
Agency Securities
Asset:
Asset, fair value	19,999
Repo Securities
Asset:
Asset, fair value	12,000
Asset Backed Securities
Asset:
Asset, fair value	10,021
Level 1
Asset:
Asset, fair value	204,874
Level 1 | Money Market Fund
Asset:
Asset, fair value	156,655	$ 10,078
Level 1 | U.S. Treasury Shares
Asset:
Asset, fair value	48,219
Level 2
Asset:
Asset, fair value	324,665
Level 2 | Corporate Bonds
Asset:
Asset, fair value	147,472
Level 2 | Commercial Paper
Asset:
Asset, fair value	135,173
Level 2 | Agency Securities
Asset:
Asset, fair value	19,999
Level 2 | Repo Securities
Asset:
Asset, fair value	12,000
Level 2 | Asset Backed Securities
Asset:
Asset, fair value	$ 10,021