Fair Value Measurements and Marketable Securities - Summary of Financial Assets and Liabilities Measured at Fair Value Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Cash equivalents:
Total cash equivalents	$ 35,890
Marketable securities:
Total marketable securities	90,745
Total cash equivalents and marketable securities	126,635	$ 83,121
Derivative liabilities:
Derivative liabilities	223	223
Anti- dilution Rights [Member]
Derivative liabilities:
Derivative liabilities	223	223
Money Market Funds [Member]
Cash equivalents:
Total cash equivalents	29,896	83,121
U.S. Government Securities [Member]
Cash equivalents:
Total cash equivalents	5,994
Marketable securities:
Total marketable securities	38,761
U.S. Corporate Bonds [Member]
Marketable securities:
Total marketable securities	35,594
U.S. Commercial Paper [Member]
Marketable securities:
Total marketable securities	16,390
Level 2 [Member]
Cash equivalents:
Total cash equivalents	35,890
Marketable securities:
Total marketable securities	90,745
Total cash equivalents and marketable securities	126,635	83,121
Level 2 [Member] | Money Market Funds [Member]
Cash equivalents:
Total cash equivalents	29,896	83,121
Level 2 [Member] | U.S. Government Securities [Member]
Cash equivalents:
Total cash equivalents	5,994
Marketable securities:
Total marketable securities	38,761
Level 2 [Member] | U.S. Corporate Bonds [Member]
Marketable securities:
Total marketable securities	35,594
Level 2 [Member] | U.S. Commercial Paper [Member]
Marketable securities:
Total marketable securities	16,390
Level 3 [Member]
Derivative liabilities:
Derivative liabilities	223	223
Level 3 [Member] | Anti- dilution Rights [Member]
Derivative liabilities:
Derivative liabilities	$ 223	$ 223