Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Number of Stock Options Outstanding
Shares, beginning balance	10,480,891	6,468,743	7,426,417
Shares, granted	4,069,813	6,086,984	1,720,242
Shares, exercised	(1,126,788)	(593,173)	(2,031,903)
Shares, cancelled	(2,111,698)	(1,481,663)	(646,013)
Shares, ending balance	11,312,218	10,480,891	6,468,743	7,426,417
Shares, exercisable as of December 31, 2014	4,612,219
Shares, vested and expected to vest as of December 31, 2014	9,839,708
Weighted-Average Exercise Price
Weighted average exercise price, beginning balance	$ 3.25	$ 4.96	$ 3.57
Weighted average exercise price, granted	1.84	2.43	7.53
Weighted average exercise price, exercised	1.30	2.00	1.31
Weighted average exercise price, cancelled	3.82	6.75	7.40
Weighted average exercise price, ending balance	2.83	$ 3.25	$ 4.96	$ 3.57
Weighted average exercise price, exercisable as of December 31, 2014	3.55
Weighted average exercise price, vested and expected to vest as of December 31, 2014	$ 2.83
Weighted-Average Remaining Contractual Life (Years)
Weighted average remaining contractual life (years), balance	8 years	7 years 4 months 24 days	6 years 8 months 12 days	6 years 4 months 24 days
Aggregate Intrinsic Value
Aggregate intrinsic value, beginning balance	$ 1,073	$ 8,991	$ 40,040
Aggregate intrinsic value, ending balance	$ 2,450	$ 1,073	$ 8,991	$ 40,040