Fair Value of Financial Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Cash equivalents	$ 74,883	$ 8,134
Liabilities:
Non-financial liability		5,148
Preferred stock warrant liability
Liabilities:
Non-financial liability		4,947
Derivative liability
Liabilities:
Non-financial liability		201
Level 1
Assets:
Cash equivalents	0	0
Liabilities:
Non-financial liability		0
Level 1 | Preferred stock warrant liability
Liabilities:
Non-financial liability		0
Level 1 | Derivative liability
Liabilities:
Non-financial liability		0
Level 2
Assets:
Cash equivalents	74,883	8,134
Liabilities:
Non-financial liability		0
Level 2 | Preferred stock warrant liability
Liabilities:
Non-financial liability		0
Level 2 | Derivative liability
Liabilities:
Non-financial liability		0
Level 3
Assets:
Cash equivalents	$ 0	0
Liabilities:
Non-financial liability		5,148
Level 3 | Preferred stock warrant liability
Liabilities:
Non-financial liability		4,947
Level 3 | Derivative liability
Liabilities:
Non-financial liability		$ 201