Fair Value Measurements - Common Stock Warrants (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Investment warrants, exercise price (in dollars per share)	$ 0.09
Warrants And rights outstanding term	3 years 8 months
Fair value of common stock warrants	$ 14.13	$ 19.21
Warrant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock warrants outstanding (in shares)	354,350	354,350