| 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 |
| Operating activities | | | | | | | | |
| Net loss | -51.8 | -194.9 | -37.3 | -26.3 | -25.9 | -27.4 | -36.1 | -32.6 |
| |
| Reconciliation of net loss to net cash used in operating activities: | | | | | | | | |
| Stock-based compensation expense | 4.6 | 3.4 | 1.4 | | 1.8 | 2.0 | 2.2 | 1.7 |
| Other non-cash items | 0.1 | 0.0 | 0.1 | 1.6 | | | | |
| Changes in assets and liabilities: | | | | | | | | |
| Prepaid expenses and other assets | -1.6 | 0.1 | 1.3 | -1.3 | -0.2 | -0.1 | -0.2 | -0.5 |
| Lease liability - operating lease | 0.6 | -0.7 | | | | | | |
| Accounts payable | -0.8 | -2.0 | 0.3 | -0.2 | 0.7 | -0.6 | 0.1 | 0.2 |
| Accrued expenses | 4.4 | -1.3 | 1.6 | -0.2 | 0.4 | -1.2 | 0.4 | 1.0 |
| Net cash used in operating activities | -43.5 | -30.2 | -32.5 | -26.6 | -23.5 | -26.8 | -32.9 | -29.8 |
| |
| Investing activities | | | | | | | | |
| Purchases of short-term investments | -396.1 | -78.0 | -62.8 | -155.2 | -70.4 | | -45.5 | |
| Sales and maturities of short-term investments | 229.0 | 59.0 | 69.1 | 151.1 | 48.1 | 14.9 | 30.5 | |
| Purchases of property and equipment | -0.1 | 0.0 | 0.0 | 0.0 | | -0.1 | -0.5 | -0.4 |
| Net cash provided by (used in) investing activities | -167.1 | -12.6 | 6.3 | -4.1 | -21.9 | 14.9 | -15.5 | -0.4 |
| |
| Financing activities | | | | | | | | |
| Proceeds from exercise of stock options | 0.1 | 0.0 | 0.0 | 0.1 | 0.0 | 0.0 | 0.5 | 0.1 |
| Net cash provided by financing activities | 144.7 | 104.3 | 40.9 | 25.6 | 44.3 | 4.7 | 9.2 | 78.3 |
| |
| Supplemental disclosure of non-cash transactions: | | | | | | | | |
| Conversion of Series X Preferred Stock into common stock | | 168.9 | | | | | | |