Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Line Items]
Loss from operations	$ (7,101)	$ (4,911)	$ (4,651)	$ (3,839)	$ (4,273)	$ (3,365)	$ (2,461)	$ (2,240)
Net loss	$ (6,480)	$ (4,910)	$ (4,556)	$ (3,838)	$ (4,225)	$ (3,368)	$ (2,497)	$ (2,228)	$ (19,784)	$ (12,317)	$ (17,068)
Net loss per share - basic and dilutive (in dollars per share)	$ (0.10)	$ (0.08)	$ (0.08)	$ (0.07)	$ (0.08)	$ (0.08)	$ (0.06)	$ (0.05)	$ (0.32)	$ (0.27)	$ (0.49)
Weighted average common share - basic and dilutive (in shares)	65,483,336	58,543,528	58,543,528	58,324,260	46,690,621	44,654,414	44,654,414	44,601,396	61,945,356	45,667,813	34,896,592