| In millions, except per share items | Dec-31-22 | Dec-31-21 | Dec-31-20 | Dec-31-19 | Dec-31-18 | Dec-31-17 |
| | 10-K | 10-K | 10-K | 10-K | 10-K | S-1/A |
| Assets | | | | | | |
| Current assets: | | | | | | |
| Cash and cash equivalents | 59.3 | 171.4 | 102.0 | 101.6 | 185.9 | |
| Marketable securities | 289.0 | 236.7 | 126.2 | | | |
| Accounts receivable | 0.5 | 2.3 | 7.3 | 3.5 | | |
| Prepaid expenses and other current assets | 13.5 | 7.9 | 3.9 | 3.1 | 1.2 | |
| Total current assets | 362.2 | 418.3 | 239.4 | 243.6 | 187.1 | |
| Operating lease right-of-use assets | 3.5 | 4.8 | | | | |
| Property and equipment, net | 2.1 | 2.6 | 2.6 | 3.4 | 1.8 | |
| Restricted cash | 0.6 | 0.6 | 0.3 | 0.3 | 0.3 | |
| Other assets | 0.2 | 0.0 | 0.1 | 0.1 | 0.1 | |
| Total assets | 368.6 | 426.3 | 242.4 | 247.4 | 189.3 | |
| |
| Liabilities | | | | | | |
| Current liabilities: | | | | | | |
| Accounts payable | 3.5 | 4.8 | 3.8 | 5.2 | 1.7 | |
| Accrued expenses | 13.2 | 12.8 | 10.2 | 15.0 | 7.0 | |
| Deferred revenue, current portion | 0.5 | 20.6 | 25.3 | | | |
| Total current liabilities | 17.1 | 38.3 | 39.4 | 35.4 | 34.9 | |
| |
| Long-term liabilities: | | | | | | |
| Operating lease liability, net of current portion | 2.3 | 3.8 | | | | |
| Total liabilities | 19.5 | 89.6 | 97.2 | 106.5 | 102.0 | |
| |
| Commitments and contingencies (Note 10) | | | | | | |
| Stockholders’ Equity | | | | | | |
| Common shares | 0.0 | 0.0 | 0.0 | 0.0 | | |
| Additional paid‑in capital | 649.5 | 575.2 | 287.7 | 238.4 | 1.6 | |
| Accumulated deficit | -297.1 | -238.1 | -142.5 | -97.5 | -54.2 | |
| Accumulated other comprehensive loss | -3.3 | -0.5 | 0.0 | 0.0 | | |
| Total stockholders’ equity | 349.1 | 336.7 | 145.2 | 140.9 | -52.6 | |
| Total liabilities and stockholders’ equity | 368.6 | 426.3 | 242.4 | 247.4 | 189.3 | |