Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Income tax benefit	$ 0	$ 0
Provision to income tax expense for TCJA		$ 18,894,000
Impact to effective tax rate		0.00%
Total revenue	5,991,000	$ 2,646,000
Government research and development tax credits	6,917,000	5,662,000
Unrecognized tax benefits	0	0
Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Government research and development tax credits	6,917,000
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	165,381,000
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	164,873,000
ASU 2014-09 (Topic 606)
Operating Loss Carryforwards [Line Items]
Total revenue		506,000
Impact on operating loss carryforwards		0
License and collaboration revenue
Operating Loss Carryforwards [Line Items]
Total revenue	$ 5,982,000	2,271,000
License and collaboration revenue | ASU 2014-09 (Topic 606)
Operating Loss Carryforwards [Line Items]
Total revenue		$ 506,000