Commitments and Contingencies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Operating lease ending date	Jul. 31, 2019
Operating lease beginning date	Mar. 01, 2014
Sub-lease annual rental income	$ 71,000
Rent expense	161,000	$ 170,000
Accrued costs of Settlement Agreement	$ 505,000