Short-term Investments - Summary of Hierarchy for Assets Measured at Fair Value on a Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
U.S. Government Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Fair Value	$ 19,964
U.S. T-bills [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Fair Value	5,000
U.S. Agency Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Fair Value	1,000
Money market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Fair Value	9,081	$ 61,134
Quoted Market Prices for Identical Assets (Level 1) [Member] | U.S. Government Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Fair Value	19,964
Quoted Market Prices for Identical Assets (Level 1) [Member] | U.S. T-bills [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Fair Value	5,000
Quoted Market Prices for Identical Assets (Level 1) [Member] | Money market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Fair Value	9,081	$ 61,134
Significant Other Observable Inputs (Level 2) [Member] | U.S. Agency Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Fair Value	$ 1,000