Stockholders' Equity, Equity Incentive Plans and Warrants (Summary of Stock Option Activity) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Number of shares
Options outstanding at beginning of period (in shares)	9,850,498
Options granted (in shares)	2,370,500
Options exercised (in shares)	(1,184,829)
Options forfeited (in shares)	(554,079)
Options expired (in shares)	(529,146)
Options outstanding at end of period (in shares)	9,952,944
Options vested and expected to vest (in shares)	9,578,949
Options exercisable (in shares)	6,141,839
Weighted average exercise price
Options outstanding at beginning of period, weighted average exercise price	$ 2.58
Options granted, weighted average exercise price	$ 4.04
Options exercised, weighted average exercise price	$ 2.20
Options forfeited, weighted average exercise price	$ 2.53
Options expired, weighted average exercise price	$ 8.89
Options vested and expected to vest, weighted average exercise price	$ 2.61
Options exercisable, weighted average exercise price	$ 2.48
Options outstanding at end of period, weighted average exercise price	$ 2.64
Options outstanding, weighted average remaining contractual term	4 years 4 months 4 days
Options outstanding, aggregate intrinsic value	$ 18,369,137
Options vested and expected to vest, weighted average remaining contractual term	4 years 3 months 7 days
Options vested and expected to vest, aggregate intrinsic value	17,994,450
Options exercisable, weighted average remaining contractual term	3 years 6 months 7 days
Options exercisable, aggregate intrinsic value	$ 12,604,603