Summary of Significant Accounting Policies - Weighted-Average Anti-Dilutive Securities Not Included in Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 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)	5,179	8,228
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)	330	3,156
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)	1,860	1,204
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)	87	39
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)	2,141	3,829
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)	761