Accrued Liabilities - Components of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued employee compensation and related taxes	$ 1,704	$ 1,538
Accrued outsourced clinical trials and preclinical studies	1,129	581
Accrued legal fees and expenses	376	185
Accrued other professional service fees	246	232
Deferred and accrued facility lease obligations	124	74
Value of liability-classified stock purchase warrants	82	82
Accrued equipment and lab materials	33	197
Other accrued liabilities	174	102
Total accrued liabilities	$ 3,868	$ 2,991