| 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 | | -51.1 | -105.7 | -95.5 | -91.9 | -61.5 | -49.5 | -45.0 |
| Adjustments to reconcile net loss to net cash used in operating activities: | | | | | | | | |
| Depreciation and amortization | | 1.0 | 1.0 | 1.0 | 1.0 | 1.1 | 1.1 | 1.1 |
| Discount accretion and premium amortization on marketable securities | | 2.1 | 1.7 | 1.2 | 0.6 | 0.3 | 0.4 | 0.5 |
| Equity‑based compensation | | 25.8 | 23.5 | 21.2 | 18.6 | 15.8 | 13.0 | 11.1 |
| Change in operating assets and liabilities: | | | | | | | | |
| Accounts receivable | | 0.3 | 0.7 | 5.0 | -1.0 | 2.2 | 0.9 | -3.8 |
| Prepaid expenses and other current assets | | -4.1 | -1.1 | -4.0 | -1.6 | -1.7 | -2.8 | -0.8 |
| Other assets | | -0.1 | -0.1 | 0.1 | 0.1 | 0.1 | 0.1 | 0.1 |
| Accounts payable | | -1.4 | 1.2 | 1.0 | 0.0 | 1.4 | -2.3 | -1.1 |
| Accrued expenses | | 2.0 | 1.6 | 1.5 | 1.0 | -0.3 | 1.4 | 3.5 |
| Deferred revenue | | -70.9 | -14.3 | -14.8 | -3.9 | -6.2 | -9.5 | -11.5 |
| Net cash used in operating activities | | -96.1 | -91.2 | -83.5 | -77.3 | -49.0 | -47.4 | -46.0 |
| |
| Cash flows from investing activities: | | | | | | | | |
| Purchases of marketable securities | | -309.3 | -277.0 | -244.2 | -274.0 | | -148.4 | -145.3 |
| Proceeds from maturities of marketable securities | | 80.6 | 65.1 | 132.0 | | 185.0 | 207.0 | 154.0 |
| Purchases of property and equipment | | -0.9 | -1.0 | -1.0 | -0.6 | -0.4 | -0.3 | -0.5 |
| Net cash used in investing activities | | -229.6 | -213.0 | -113.2 | -129.7 | 36.2 | 58.3 | 8.2 |
| |
| 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 | | | 1.1 | | 1.1 | | 1.7 | |
| Proceeds from issuance of common shares, net | | | | | 17.4 | | | |
| Proceeds from issuance of common shares upon stock option exercises | | 9.0 | 9.1 | 9.5 | 10.3 | 7.1 | 5.9 | 3.5 |
| Net cash provided by financing activities | | 67.8 | 28.4 | 266.3 | 293.3 | 279.0 | 278.1 | 38.3 |
| |
| Non-cash activities: | | | | | | | | |
| Purchases of property and equipment included in accounts payable and accrued expenses | | 0.4 | | | 0.2 | 0.0 | 0.0 | 0.0 |
| Amounts from exercise of stock options included in prepaid expenses and other current assets | | 0.0 | -0.1 | | | 0.0 | 0.1 | |
| Unpaid issuance costs for the at-the-market and secondary offerings included in accounts payable and accrued expenses | | 0.1 | | | | | | |