Summary of Unvested Stock Option Activity (Detail) - $ / shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Weighted Average Grant Date Fair Value
Granted	$ 1.02	$ 1.93	$ 1.38	$ 4.32
Stock Options
Shares
Beginning Balance			846
Granted			487
Vested			(337)
Forfeited or cancelled			(102)
Ending Balance	894		894
Weighted Average Grant Date Fair Value
Beginning Balance			$ 3.14
Granted			1.38
Vested			3.40
Forfeited or cancelled			2.58
Ending Balance	$ 2.15		$ 2.15