In millions, except per share items | Dec-31-22 | Dec-31-21 | Dec-31-20 | Dec-31-19 | Dec-31-18 | Dec-31-17 | Dec-31-16 | Dec-31-15 |
| 10-K | 10-K | 10-K | 10-K/A | 10-K | 10-K | 10-K | 10-K |
Assets | | | | | | | | |
Current assets: | | | | | | | | |
Cash and cash equivalents | 20.5 | 86.5 | 24.9 | 9.9 | 15.3 | 16.4 | 23.6 | 62.8 |
Short-term investments | 205.9 | 39.0 | 20.0 | 26.3 | 22.3 | | | |
Prepaid expenses and other current assets | 1.3 | 1.6 | 1.4 | 2.7 | 1.3 | 1.1 | 1.0 | 0.8 |
Total current assets | 227.7 | 127.1 | 46.3 | 39.0 | 38.9 | 17.5 | 39.5 | 63.6 |
Right-of-use asset | 0.9 | 0.4 | 1.0 | 2.3 | | | | |
Other assets | 2.0 | 0.0 | 0.2 | 0.5 | 0.3 | | | |
Total assets | 230.6 | 127.5 | 47.5 | 41.8 | 39.2 | 17.9 | 40.2 | 64.2 |
|
Liabilities and stockholders' equity | | | | | | | | |
Current liabilities: | | | | | | | | |
Accounts payable | 0.8 | 1.6 | 1.5 | 1.2 | 1.4 | 0.8 | 1.4 | 1.3 |
Accrued expenses | 7.7 | 3.3 | 4.2 | 2.6 | 2.8 | 2.4 | 3.7 | 3.3 |
Current portion of operating lease liabilities | 0.6 | 0.4 | 0.6 | 1.2 | | | | |
Total current liabilities | 9.1 | 5.2 | 6.4 | 5.0 | 4.2 | 6.0 | 8.3 | 7.8 |
Long term portion of operating lease liabilities | 0.4 | | | | | | | |
Total liabilities | 9.4 | 5.2 | 6.8 | 6.1 | 4.2 | 6.1 | 11.1 | 13.7 |
|
Commitments | | | | | | | | |
Stockholders' equity: | | | | | | | | |
Preferred stock, $0.001 par value per share, 4,908,620 shares authorized and no shares issued and outstanding | | | | | | | | |
Common stock | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
Additional paid-in capital | 632.5 | 481.7 | 301.6 | 259.3 | 232.2 | 183.2 | 173.1 | 158.5 |
Accumulated other comprehensive loss | -0.1 | | | | 0.0 | | 0.0 | |
Accumulated deficit | -507.6 | -455.8 | -260.9 | -223.6 | -197.3 | -171.4 | -144.1 | -108.0 |
Total stockholders' equity | 221.2 | 122.3 | 40.7 | 35.7 | 34.9 | 11.8 | 29.1 | 50.5 |
Total liabilities and stockholders' equity | 230.6 | 127.5 | 47.5 | 41.8 | 39.2 | 17.9 | 40.2 | 64.2 |