Earnings Per Share Applicable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Net loss		$ (7,755)	$ (8,337)	$ (8,050)	$ (8,443)	$ (16,092)	$ (16,493)
Basic and diluted loss per common share		$ (0.14)		$ (0.15)		$ (0.29)	$ (0.30)
Basic and diluted weighted average common shares outstanding	[1]	55,539		54,892		55,433	54,875
Stock Option [Member]
Potentially dilutive securities (in thousands)	[2]	10,174		8,559		10,174	8,559
Warrant [Member]
Potentially dilutive securities (in thousands)	[2]	140		470		140	470
Convertible Notes [Member]
Potentially dilutive securities (in thousands)	[2]	2,192		0		2,192	0

[1] Includes approximately 0.2 million and 0.2 million nonvested restricted stock for the periods ending June 30, 2019 and June 30, 2018, respectively, which are participating securities that feature voting and dividend rights.

[2] Excluded from the computation of loss per share as their impact is antidilutive.