Net Loss Per Share - Weighted-Average Anti-Dilutive Securities Not Included in Diluted Net Loss Per Share (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]
Anti-dilutive securities not included in diluted net loss per share (in shares)	5,940	8,969	4,929	6,295
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in diluted net loss per share (in shares)	446	1,444	390	1,298
Unvested restricted share awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in diluted net loss per share (in shares)		55		69
Warrants to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in diluted net loss per share (in shares)	2,530	87	1,097	1,132
Series A Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in diluted net loss per share (in shares)	2,022	7,383	2,262	3,796
Convertible Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in diluted net loss per share (in shares)	942		1,180