Income Taxes - Schedule of Tax Effect of Significant Temporary Differences Representing Deferred Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Components of Deferred Tax Assets and Liabilities [Abstract]
Net operating loss (“NOL”) and credit carryforwards	$ 37,252	$ 43,765
Capitalized research and development costs	16,115	22,775
Other	1,722	1,963
Valuation allowance	(55,089)	(68,503)
Deferred Tax Assets, Net	$ 0	$ 0