Property and Equipment - Property and Equipment and Related Accumulated Depreciation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 853	$ 867
Less accumulated depreciation and amortization	(331)	(71)
Total property and equipment, net	$ 522	796
Lab equipment
Property, Plant and Equipment [Line Items]
Estimated useful life (in years)	5 years
Property and equipment, gross	$ 443	457
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful life (in years)	4 years
Property and equipment, gross	$ 16	16
Computer equipment
Property, Plant and Equipment [Line Items]
Estimated useful life (in years)	3 years
Property and equipment, gross	$ 73	73
Software
Property, Plant and Equipment [Line Items]
Estimated useful life (in years)	3 years
Property and equipment, gross	$ 28	28
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 293	$ 293