| In millions, except per share items | Mar-31-22 | Dec-31-21 | Sep-30-21 | Mar-31-21 | Dec-31-20 | Sep-30-20 | Jun-30-20 | Jun-05-20 |
| | 10-Q | 10-K | 10-Q | 10-Q | 10-K | 10-Q | 10-Q | 8-K |
| Cash flows from operating activities: | | | | | | | | |
| Net loss | -33.4 | -7.0 | -8.7 | -8.1 | -8.1 | -25.3 | | |
| Adjustments to reconcile net loss to net cash used in operating activities: | | | | | | | | |
| Depreciation and amortization | 0.6 | 0.4 | 0.4 | 0.4 | 0.4 | 0.5 | 0.3 | |
| Stock-based compensation | 0.4 | 0.5 | 0.5 | 0.4 | 0.4 | 0.4 | 0.4 | |
| Change in operating lease right-of-use assets | 0.3 | 0.3 | | 0.3 | 0.3 | 0.2 | 0.2 | |
| Accretion of asset retirement obligation | 0.2 | | | 0.0 | | 0.1 | | |
| Effective interest on financing obligations | 0.1 | 0.1 | | 0.1 | 0.2 | 0.1 | 0.2 | |
| Capitalized effective interest | 0.0 | -0.1 | | -0.1 | -0.1 | -0.1 | -0.1 | |
| Loss on impairment of long-lived assets | 25.9 | | | | | | | |
| Change in operating assets and liabilities: | | | | | | | | |
| Accounts receivable, prepaid expenses and other assets | -1.5 | 0.4 | | -1.1 | 0.2 | 0.0 | -0.1 | |
| Accounts payable | 0.7 | 0.4 | | 0.3 | 0.4 | -0.4 | -0.9 | |
| Accrued compensation and other accrued liabilities | -1.1 | -0.5 | | 0.7 | 0.3 | -0.3 | | |
| Operating lease liabilities | -0.4 | -0.4 | | -0.3 | -0.3 | -0.3 | -0.3 | |
| Net cash used in operating activities | -8.5 | -5.8 | -7.8 | -7.5 | -6.3 | -9.0 | -8.0 | |
| |
| Cash flows from investing activities: | | | | | | | | |
| Purchases of property and equipment | -0.1 | -0.4 | | -3.2 | -0.8 | -3.0 | -3.3 | |
| Net cash used in investing activities | -0.1 | -0.4 | | -3.2 | -0.8 | -3.0 | -3.3 | |
| |
| Cash flows from financing activities: | | | | | | | | |
| Proceeds from public offering of securities | | | | | | | | |
| Proceeds from warrant exercises | | | | | | -2.6 | 0.9 | |
| Principal payments on financing obligations | | | | | | | | |
| Net cash provided by financing activities | 11.1 | 0.1 | | 2.3 | -1.2 | 45.0 | 3.3 | |
| Net increase (decrease) in cash, cash equivalents and restricted cash | 2.5 | | | -8.4 | | | | |
| |
| Cash and cash equivalents | | | | | | | | |
| Restricted cash | | | | | | | | |
| |
| Supplemental cash flow information: | | | | | | | | |
| Cash paid for interest | 0.1 | 0.1 | | 0.2 | 0.2 | 0.2 | 0.2 | |
| Non-cash investing and financing activities: | | | | | | | | |
| Acquisition of property and equipment under accounts payable and other accrued liabilities | 1.5 | 0.2 | | 1.8 | -0.1 | -2.8 | -2.5 | |
| Accrued offering costs | 0.2 | | | 0.1 | | 0.4 | | |
| Asset retirement obligation | | | | 0.1 | | | | |