Significant Accounting Policies - Summary of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total stock-based expenses	$ 1,381	$ 4,013	$ 2,529
Research and development expense
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total stock-based expenses	404	1,455	1,032
General and administrative expense
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total stock-based expenses	$ 977	$ 2,558	$ 1,497