Fair Value Measurements - Summary of Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investments:
Investments	$ 112,986	$ 29,690
Fair Value, Measurements, Recurring [Member]
Investments:
Investments	146,830	64,343
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	18,846	25,510
Commercial Paper [Member]
Investments:
Investments	6,245	11,957
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	4,998	4,243
Investments:
Investments	6,245	11,957
Corporate Bonds [Member]
Investments:
Investments	2,021	17,733
Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents		4,900
Investments:
Investments	2,021	17,733
Government Securities [Member]
Investments:
Investments	104,720
Government Securities [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	10,000
Investments:
Investments	104,720
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	18,846	25,510
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	18,846	25,510
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	127,984	38,833
Level 2 [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	4,998	4,243
Investments:
Investments	6,245	11,957
Level 2 [Member] | Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents		4,900
Investments:
Investments	2,021	$ 17,733
Level 2 [Member] | Government Securities [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	10,000
Investments:
Investments	$ 104,720