Fair Value Measurements - Summary of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investments:
Investments	$ 34,492
Fair Value, Measurements, Recurring [Member]
Investments:
Investments	64,121	$ 39,982
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	24,134	39,982
Commercial Paper [Member]
Investments:
Investments	17,920
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	5,495
Investments:
Investments	17,920
Corporate Bonds [Member]
Investments:
Investments	15,029
Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	15,029
Government Securities [Member]
Investments:
Investments	545
Government Securities [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	545
Treasury Bonds [Member]
Investments:
Investments	998
Treasury Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	998
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	24,134	39,982
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	24,134	$ 39,982
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	39,987
Level 2 [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	5,495
Investments:
Investments	17,920
Level 2 [Member] | Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	15,029
Level 2 [Member] | Government Securities [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	545
Level 2 [Member] | Treasury Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	$ 998