Share-Based Payments - Summary of Stock-based Compensation Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total stock-based expenses	$ 664	$ 636
Research and Development Expense [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total stock-based expenses	280	355
General and Administrative Expense [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total stock-based expenses	$ 384	$ 281