Fair Value of Financial Assets and Liabilities - Schedule of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash equivalents:
Cash and cash equivalents	$ 78,977
Marketable securities:
Marketable securities	10,089
Cash and cash equivalents	78,977
Total assets		$ 114,592
Commercial Paper [Member]
Marketable securities:
Marketable securities	8,091
Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash and cash equivalents	78,977
Marketable securities:
Cash and cash equivalents	78,977
Total assets		114,592
Fair Value, Inputs, Level 2 [Member]
Marketable securities:
Marketable securities	10,089
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Marketable securities:
Marketable securities	8,091
Money Market Funds [Member]
Cash equivalents:
Cash and cash equivalents	78,977	114,592
Marketable securities:
Cash and cash equivalents	78,977	114,592
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash and cash equivalents	78,977	114,592
Marketable securities:
Cash and cash equivalents	78,977	$ 114,592
Corporate Debt Securities [Member]
Marketable securities:
Marketable securities	1,998
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Marketable securities:
Marketable securities	$ 1,998