Stockholders' Equity - Stock Option Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value	$ 8,233,220	$ 6,467,294	$ 7,072,923
Unrecognized compensation cost, net of forfeitures	$ 4,200,000	$ 8,600,000
Weighted average period to recognize stock compensation expense		1 year 4 months 24 days
Options
Outstanding, Beginning Balance	5,686,815	5,096,674	5,246,465
Granted during period	1,100,997	1,126,672	1,065,386
Exercised during period	(129,638)	(146,046)	(399,654)
Expired during period	(28,369)	(156,160)	(244,121)
Forfeited during period	(182,211)	(234,325)	(571,402)
Outstanding, Ending Balance	6,447,594	5,686,815	5,096,674
Weighted-Average Exercise Price
Outstanding, Beginning Balance	$ 6.87	$ 7.29	$ 6.76
Granted during period	3.21	5.23	10.54
Exercised during period	2.01	2.60	3.23
Expired during period	7.65	11.03	9.17
Forfeited during period	4.45	8.05	10.49
Outstanding, Ending Balance	$ 6.41	$ 6.87	$ 7.29
Additional disclosures
Options, Exercisable	4,666,628	3,391,069	2,695,278
Weighted Average Exercise Price, Exercisable	$ 6.76	$ 6.46	$ 5.89
Weighted Average Remaining Contractual Term	6 years 2 months 1 day	6 years 6 months 7 days	6 years 10 months 13 days
Weighted Average Remaining Contractual Term exercisable at end of period	5 years 4 months 2 days	5 years 4 months 13 days	5 years 8 months 5 days
Aggregate Intrinsic Value	$ 33,283	$ 1,432,306	$ 6,329,518
Aggregate Intrinsic Value, exercisable at end of period	$ 33,283	$ 1,333,875	$ 4,986,059