Warrant Liabilities - Fair Value of Convertible Preferred Stock Warrant Liability (Detail)	Mar. 31, 2019	Dec. 31, 2018
Expected Dividend Yield [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.00
Series C Warrant [Member] | Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.90	0.90
Series C Warrant [Member] | Contractual Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement term	11 months 1 day	1 year 2 months 1 day
Series C Warrant [Member] | Risk-free Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0241	0.0260
2017 PIPE Warrant Liability [Member] | Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.86	0.75
2017 PIPE Warrant Liability [Member] | Contractual Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement term	1 year 8 months 12 days	2 years
2017 PIPE Warrant Liability [Member] | Risk-free Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0240	0.0251
2018 PIPE Warrant Liability [Member] | Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.86	0.75
2018 PIPE Warrant Liability [Member] | Contractual Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement term	4 years 8 months 12 days	5 years
2018 PIPE Warrant Liability [Member] | Risk-free Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0223	0.0251