Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Number of Stock Options Outstanding
Number of Stock Options Outstanding, Beginning Balance	11,312,218	10,480,891	6,468,743
Number of Stock Options Outstanding, Granted	2,450,500	4,069,813	6,086,984
Number of Stock Options Outstanding, Exercised	(369,173)	(1,126,788)	(593,173)
Number of Stock Options Outstanding, Cancelled	(3,344,945)	(2,111,698)	(1,481,663)
Number of Stock Options Outstanding, Ending Balance	10,048,600	11,312,218	10,480,891	6,468,743
Number of Stock Options Outstanding, Exercisable as of December 31, 2016	5,990,641
Number of Stock Options Outstanding, Vested and expected to vest as of December 31, 2016	9,284,080
Weighted-Average Exercise Price
Weighted-Average Exercise Price, Beginning Balance	$ 2.83	$ 3.25	$ 4.96
Weighted-Average Exercise Price, Granted	1.34	1.84	2.43
Weighted-Average Exercise Price, Exercised	1.69	1.30	2.00
Weighted-Average Exercise Price, Cancelled	2.50	3.82	6.75
Weighted-Average Exercise Price, Ending Balance	2.04	$ 2.83	$ 3.25	$ 4.96
Weighted-Average Exercise Price, Exercisable as of December 31, 2016	2.31
Weighted-Average Exercise Price, Vested and expected to vest as of December 31, 2016	$ 2.08
Weighted-Average Remaining Contractual Life (Years)
Weighted-Average Remaining Contractual Life (Years), Balance	7 years 2 months 12 days	8 years	7 years 4 months 24 days	6 years 8 months 12 days
Weighted-Average Remaining Contractual Life (Years), Exercisable	6 years 4 months 24 days
Weighted-Average Remaining Contractual Life (Years), Vested and expected to vest	6 years 9 months 18 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Balance	$ 15	$ 2,450	$ 1,073	$ 8,991
Aggregate Intrinsic Value, Vested and expected to vest	$ 11