Stockholders' Equity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Weighted Average Remaining Contractual Term
Vested and expected to vest	5 years 8 months 12 days	6 years 1 month 6 days	5 years 7 months 6 days
Employee Stock Option [Member]
Shares
Outstanding, Beginning balance	7,389,894	5,974,039	6,225,612
Granted	369,814	2,547,585	240,000
Exercised	(778,783)	(352,718)	(61,756)
Forfeited	(2,712,613)	(635,424)	(390,694)
Expired	(77,308)	(143,588)	(39,123)
Outstanding, Ending balance	4,191,004	7,389,894	5,974,039	6,225,612
Exercisable, December 31, 2015	3,284,406
Vested and expected to vest, December 31, 2015	4,082,212
Weighted-Average Exercise Price
Outstanding, beginning balance	$ 2.31	$ 2.52	$ 2.52
Granted	1.66	1.81	1.65
Exercised	1.25	1.29	1.21
Forfeited	2.43	2.61	2.20
Expired	3.26	3.04	2.96
Outstanding, ending balance	2.36	$ 2.31	$ 2.52	$ 2.52
Exercisable, December 31, 2015	2.54
Vested and expected to vest, December 31, 2015	$ 2.40
Weighted Average Remaining Contractual Term
Outstanding	6 years	6 years 10 months 24 days	6 years 6 months	7 years 4 months 24 days
Exercisable	5 years 3 months 18 days
Vested and expected to vest	5 years 10 months 24 days