Income Taxes - Summary of Variation of Effective Provision (Benefit) for Income Taxes from Statutory Federal Income Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal tax benefit at statutory rate	$ (8,800)	$ (9,885)	$ (11,343)
State tax, net of federal provision (benefit)	32	28	(34)
Research and development credits	(707)	(745)	(667)
Share-based compensation	(18,154)	960	1,086
Other permanent differences	814	600	325
Change in US federal Tax Rate	40,377
Foreign tax rate differential	(445)	(225)	(80)
Net operating (gains) losses not recognized	(12,859)	9,503	10,762
Release of valuation allowance associated with acquisitions			(1,312)
Total income tax provision (benefit)	$ 258	$ 236	$ (1,263)