Significant Accounting Polices - Summary of Stock-based Compensation Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total stock-based expenses	$ 2,432	$ 1,260	$ 130
Research and Development Expense [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total stock-based expenses	1,069	1,150	117
General and Administrative Expense [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total stock-based expenses	$ 1,363	$ 110	$ 13