Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income taxes
Net operating loss, or NOL, carry forwards available to offset future federal and state taxable income that will expire beginning in 2023	$ 121,800,000	$ 93,200,000
Gross deferred tax assets:
Net operating loss carryforwards	48,654,000	37,051,000
Accrued expenses	55,000	29,000
Contributions	6,000	6,000
Deferred expenses	58,000	23,000
Stock-based compensation	1,523,000	826,000
Research and development and other credits	5,258,000	4,123,000
Total gross deferred tax assets	55,554,000	42,058,000
Gross deferred tax liabilities:
Depreciation	(3,000)	(2,000)
Total gross deferred tax liabilities	(3,000)	(2,000)
Net deferred tax assets	55,551,000	42,056,000
Less: valuation allowance	(55,551,000)	(42,056,000)
Net deferred tax assets after valuation allowance	0	0
Increase in valuation allowance	13,500,000	12,300,000
Unrecognized tax benefit	0	0
Liability for uncertain tax positions	0	0
Interest and penalties accrued related to unrecognized tax benefits	$ 0	$ 0
Reconciliation of income tax expense (benefit) at the statutory federal income tax rate and income taxes
Federal income tax expense at statutory rate (as a percent)	34.00%	34.00%	34.00%
Permanent items (as a percent)	(1.20%)	(1.00%)	(5.40%)
State income tax, net of federal benefit (as a percent)	7.50%	7.10%	3.80%
R&D tax credits (as a percent)	5.20%	4.20%	4.20%
Other (as a percent)	1.60%	5.00%
Change in valuation allowance (as a percent)	(47.10%)	(49.30%)	(36.60%)
Effective income tax rate (as a percent)	0.00%	0.00%	0.00%
Research tax credit carryforward | Federal
Income taxes
Tax credit carryovers	$ 4,400,000
Research tax credit carryforward | State
Income taxes
Tax credit carryovers	$ 900,000