Equity Compensation Plans - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Number of Shares
Outstanding as of December 31, 2014 (shares)	3,955,548
Granted (shares)	1,474,178
Exercised (shares)	(40,520)
Cancelled (shares)	(1,335,601)
Outstanding as of December 31, 2015 (shares)	4,053,605	3,955,548
Stock options vested or expected to vest as of December 31, 2015 (shares)	3,742,904
Stock options exercisable as of December 31, 2015 (shares)	1,731,331	1,613,222	752,815
Weighted Average Exercise Price
Beginning balance (USD per share)	$ 2.58
Granted (USD per share)	1.44
Exercised (USD per share)	2.05
Cancelled (USD per share)	3.27
Ending balance (USD per share)	1.95	$ 2.58
Weighted-average exercise price of vested or expected to vest stock options (USD per share)	1.98
Weighted-average exercise price of exercisable stock options (USD per share)	$ 2.29
Weighted Average Remaining Contractual Life (in years)
Outstanding (in years)	8 years 2 months	7 years 6 months
Weighted Average Remaining Contractual Life (in years) of vested or expected to vest stock options	8 years 2 months
Weighted Average Remaining Contractual Life (in years) of exercisable stock options	7 years 7 months
Total Aggregate Intrinsic Value
Total aggregate intrinsic value shares outstanding	$ 0	$ 0
Total aggregate intrinsic value of vested or expected to vest stock options	0
Total aggregate intrinsic value of exercisable stock options	$ 0