Property and Equipment - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 15,632	$ 13,642
Less: B depreciation	(11,280)	(10,919)
Property and equipment, net	4,352	2,723
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	279	279
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	4,670	4,565
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	7,711	7,712
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	2,876	990
Software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 96	$ 96