Summary of Unvested Stock Option Activity (Detail) - $ / shares shares in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Weighted Average Grant Date Fair Value
Granted	$ 1.53	$ 1.45
Stock Options
Shares
Beginning Balance	522
Granted	40
Vested	(97)
Forfeited or cancelled	(56)
Ending Balance	409
Weighted Average Grant Date Fair Value
Beginning Balance	$ 2.11
Granted	1.53
Vested	3.52
Forfeited or cancelled	3.00
Ending Balance	$ 1.59