Organization and Going Concern - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Accumulated deficit	$ (712,442)	$ (657,997)
Cash and cash equivalents	$ 70,946	$ 81,053	$ 140,717	$ 42,803