In millions, except per share items | Jun-30-23 | Jun-30-22 | Mar-31-22 | Dec-31-21 | Sep-30-21 | Jun-30-21 | Mar-31-21 | Dec-31-20 |
| 10-Q | 10-Q | 10-Q | 10-K | 10-Q | 10-Q | 10-Q | 10-K |
Statement of Cash Flows [Abstract] | | | | | | | | |
Cash flows from operating activities: | | | | | | | | |
Net loss | | 26.8 | -31.5 | -21.4 | -25.0 | -27.8 | -21.3 | -17.7 |
Adjustments to reconcile net loss to net cash used in operating activities: | | | | | | | | |
Depreciation and amortization | | 0.2 | 0.3 | 0.3 | 0.2 | 0.3 | 0.3 | 0.3 |
Discount accretion and premium amortization on marketable securities | | 0.5 | 0.6 | 0.7 | 0.3 | 0.1 | 0.1 | 0.0 |
Equity‑based compensation | | 7.6 | 6.8 | 5.6 | 5.8 | 5.3 | 4.4 | 3.0 |
Change in operating assets and liabilities: | | | | | | | | |
Accounts receivable | | 1.0 | 0.3 | 0.3 | -1.3 | 1.4 | 4.6 | -5.7 |
Prepaid expenses and other current assets | | -1.2 | 1.2 | -2.7 | -1.3 | 1.7 | -1.8 | -0.2 |
Other assets | | 0.0 | -0.2 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
Operating lease right-of-use assets | | 0.3 | 0.3 | | | 0.3 | 0.3 | |
Accounts payable | | 0.3 | -1.3 | 1.8 | -2.2 | 2.9 | -1.6 | 0.8 |
Accrued expenses | | 1.0 | -3.0 | 3.2 | 0.7 | 0.7 | -3.1 | 2.7 |
Deferred revenue | | -59.2 | -1.4 | -8.5 | -1.8 | -2.5 | -1.9 | 2.4 |
Operating lease liability | | -0.5 | | | | | | |
Net cash used in operating activities | | -22.8 | -28.1 | -20.6 | -24.6 | -17.9 | -20.4 | -14.4 |
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Cash flows from investing activities: | | | | | | | | |
Purchases of marketable securities | | -32.2 | -45.1 | -20.4 | -211.6 | | -12.2 | -50.2 |
Proceeds from maturities of marketable securities | | 35.5 | 19.1 | 26.0 | | 20.0 | 86.0 | 39.0 |
Purchases of property and equipment | | 0.0 | 0.0 | -0.5 | -0.4 | -0.2 | -0.1 | -0.1 |
Net cash used in investing activities | | 3.3 | -26.1 | 5.2 | -212.0 | 19.8 | 73.7 | -11.3 |
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Cash flows from financing activities: | | | | | | | | |
Proceeds from issuance of common shares and pre-funded warrants through the private placement | | | | | 17.4 | | | |
Proceeds from issuance of common shares under Employee Stock Purchase Plan | | | 0.6 | | 0.5 | | 0.6 | |
Proceeds from issuance of common shares, net | | | | | 17.4 | | | |
Proceeds from issuance of common shares upon stock option exercises | | 1.6 | 2.1 | 1.6 | 3.7 | 1.7 | 2.5 | 2.3 |
Net cash provided by financing activities | | 40.9 | 2.6 | 2.7 | 21.6 | 1.5 | 240.6 | 29.7 |
Net increase (decrease) in cash, cash equivalents and restricted cash | | -30.3 | | | | | | |
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Non-cash activities: | | | | | | | | |
Purchases of property and equipment included in accounts payable and accrued expenses | | 0.2 | | | 0.1 | 0.0 | 0.0 | 0.0 |
Amounts from exercise of stock options included in prepaid expenses and other current assets | | 0.0 | 0.1 | | | -0.1 | 0.1 | |
Unpaid issuance costs for the at-the-market and secondary offerings included in accounts payable and accrued expenses | | 0.1 | | | | | | |