Income Taxes - U S federal statutory income tax rate (Details) - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2020 |
Dec. 31, 2019 |
|
| Income Taxes | ||
| Income tax benefits - operating losses incurred | $ 0.0 | $ 0.0 |
| Income tax benefits - research and development tax credits | $ 0.0 | $ 0.0 |
| Federal statutory income tax rate | (21.00%) | (21.00%) |
| State taxes, net of federal benefit | (6.20%) | (6.20%) |
| Federal and state research and development tax credits | (5.90%) | (6.00%) |
| Stock-based compensation expense | 0.40% | 0.70% |
| Other | 0.50% | 0.50% |
| Increase in deferred tax asset valuation allowance | 32.20% | 32.00% |