Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Income Taxes
Net operating loss, or NOL, carry forwards available to offset future federal and state taxable income that will expire beginning in 2023	$ 68,000,000	$ 59,000,000
Income taxes
Exchange of research and development credits (as a percent)	65.00%
Net benefit from the estimated proceeds from the exchange of the research and development credit	2,000	28,000
Gross deferred tax assets:
Net operating loss carryovers	26,505,000	23,006,000
Accrued expenses		18,000
Contributions	5,000	1,000
Stock-based compensation	211,000	143,000
Research and development and other credits	3,075,000	2,693,000
Total gross deferred tax assets	29,796,000	25,861,000
Gross deferred tax liabilities:
Depreciation	(1,000)	(2,000)
Total gross deferred tax liabilities	(1,000)	(2,000)
Net deferred tax assets	29,795,000	25,859,000
Less: valuation allowance	(29,795,000)	(25,859,000)
Increase in valuation allowance	3,900,000	2,300,000
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%
Permanent items (as a percent)	(5.40%)	(0.30%)
State income tax, net of federal benefit (as a percent)	3.80%	5.00%
State refundable credit (as a percent)	0.00%	0.70%
R&D tax credits (as a percent)	4.20%	(3.30%)
Change in valuation allowance (as a percent)	(36.60%)	(35.40%)
Effective income tax rate (as a percent)	0.00%	0.70%
Uncertain tax position	0
Research tax credit carryforward | Federal
Income taxes
Tax credit carryovers	2,600,000
Research tax credit carryforward | State
Income taxes
Tax credit carryovers	$ 400,000