Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net (loss)	$ (5,980)	$ (6,561)	$ (8,491)	$ (9,352)	$ (11,093)	$ (4,351)	$ (5,673)	$ (5,379)	$ (30,384)	$ (26,496)	$ (15,278)
Weighted-average shares used to compute basic net loss per share (in shares)									57,172,659	55,932,641	55,135,657
Effect of dilutive options, unvested shares of restricted stock awards and unvested restricted stock units (in shares)									0	0	0
Weighted-average shares used to compute diluted net (loss) income per share (in shares)									57,172,659	55,932,641	55,135,657
Net loss per share:
Basic (in dollars per share)	$ (0.10)	$ (0.11)	$ (0.15)	$ (0.17)	$ (0.20)	$ (0.08)	$ (0.10)	$ (0.10)	$ (0.53)	$ (0.47)	$ (0.28)
Diluted (in dollars per share)	$ (0.10)	$ (0.11)	$ (0.15)	$ (0.17)	$ (0.20)	$ (0.08)	$ (0.10)	$ (0.10)	$ (0.53)	$ (0.47)	$ (0.28)
Options to purchase common stock
Weighted Average Number Diluted Shares Outstanding Adjustment Abstract [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									4,078,626	4,688,256	5,210,384
Unvested shares of restricted stock awards
Weighted Average Number Diluted Shares Outstanding Adjustment Abstract [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									562,936	564,652	557,982
Unvested shares of restricted stock units
Weighted Average Number Diluted Shares Outstanding Adjustment Abstract [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									2,032,202	1,765,905	1,198,290