Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Stockholders' Equity
Options, Outstanding, Beginning Balance	21,402,200
Granted	4,116,800
Forfeited	(894,377)
Expired	(407,075)
Options, Outstanding, Ending Balance	24,217,548		21,402,200
Exercisable, Ending Balance	14,734,826
Weighted Average Exercise Price, Beginning Balance	$ 2.94
Granted, Weighted Average Exercise Price	2.24	$ 1.59
Forfeited, Weighted Average Exercise Price	2.46
Expired, Weighted Average Exercise Price	11.09
Weighted Average Exercise Price, Ending Balance	2.70		$ 2.94
Exercisable, Weighted Average Exercise Price	$ 2.95
Outstanding, Ending balance, Weighted Average Remaining Contractual Term	6 years 8 months 12 days		6 years 6 months
Exercisable Ending Balance, Weighted Average Remaining Contractual Term	5 years 4 months 24 days
Outstanding, Intrinsic Value, Ending Balance	$ 3,716		$ 5,805
Exercisable, Ending Balance, Intrinsic Value	2,998
Fair value of options, vested	2,100
Unrecognized compensation cost related to nonvested stock-based compensation	$ 11,100
Weighted average remaining period over which unrecognized compensation expense will be recognized	2 years 7 months 6 days