Stock-Based Compensation - Summary of Weighted-Average and Total Estimated Grant Date Fair Values of Employee Stock Options Granted (Detail) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jul. 10, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assumptions:
Expected dividend yield		0.00%
Fair value:
Options granted to employees (in thousands)	543,650	2,105,000
Stock Option Plans [Member]
Assumptions:
Expected term (years)		4 years 9 months 18 days	5 years 3 months 18 days	5 years 2 months 12 days
Expected volatility		112.50%	110.00%	99.90%
Risk-free interest rate		2.10%	1.90%	1.70%
Expected dividend yield		0.00%	0.00%	0.00%
Fair value:
Weighted-average estimated grant date fair value per share		$ 2.68	$ 3.12	$ 6.20
Stock Option Plans [Member] | Employee stock-based compensation expense [Member]
Fair value:
Options granted to employees (in thousands)		1,280,000	750,000	531,000
Total estimated grant date fair value (in thousands)		$ 3,434	$ 2,344	$ 3,294