Net Loss per Share - Anti-Dilutive Securities (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares excluded as anti-dilutive (shares)	7,462	7,694	7,462	7,694
Shares of common stock issuable pursuant to equity awards outstanding under the Equity Incentive Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares excluded as anti-dilutive (shares)	7,462	7,621	7,462	7,621
Shares of common stock issuable upon exercise of outstanding warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares excluded as anti-dilutive (shares)	0	73	0	73