Income Taxes - U S federal statutory income tax rate (Details) - USD ($) $ in Millions |
12 Months Ended | 24 Months Ended | |
|---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2018 |
|
| Income Taxes | |||
| Income tax benefits - operating losses incurred | $ 0.0 | ||
| Income tax benefits - research and development tax credits | $ 0.0 | ||
| Federal statutory income tax rate | (21.00%) | (34.00%) | |
| State taxes, net of federal benefit | (4.80%) | (3.20%) | |
| Federal and state research and development tax credits | (2.90%) | (1.40%) | |
| Stock-based compensation expense | 4.50% | 12.80% | |
| Other | 0.50% | 0.70% | |
| Remeasurement of deferred taxes due to the Tax Cuts and Jobs Act | 11.10% | ||
| Increase in deferred tax asset valuation allowance | 23.70% | 14.00% | |