Basic and Diluted Net Loss per Common Share (Details) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded because such instruments were anti-dilutive (in shares)	6,081	18,815
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded because such instruments were anti-dilutive (in shares)	2,029	1,922
Unvested restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded because such instruments were anti-dilutive (in shares)	553	759
Potentially issuable employee stock purchase plan shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded because such instruments were anti-dilutive (in shares)	61	0
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded because such instruments were anti-dilutive (in shares)	1,819	14,515
Convertible notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded because such instruments were anti-dilutive (in shares)	1,619	1,619