| 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 | 10-K | 10-K | 10-K | 10-K |
| Investment income | | | | | | | | |
| Interest income on non-affiliate investments | | | | | | | | |
| Fee income on non-affiliate investments | | | | | | | | |
| Total investment income | 79.2 | 60.0 | 46.0 | 43.1 | 31.1 | 25.8 | 33.0 | 31.1 |
| Total investment income growth | 32.0% | 30.4% | 6.7% | 38.7% | 20.6% | -21.8% | 6.0% | 190.3% |
| Expenses | | | | | | | | |
| Interest expense | 19.2 | 12.0 | 9.7 | 8.3 | | 5.2 | 5.9 | 5.8 |
| Base management fee (Note 3) | 10.6 | 7.6 | 6.5 | | | | | |
| Performance based incentive fee (Note 3) | 7.7 | 7.1 | 5.2 | | | | | |
| Administrative fee (Note 3) | 1.7 | 1.3 | 1.0 | | | | | |
| Professional fees | 1.7 | 1.9 | 1.5 | | | | | |
| General and administrative | 1.5 | 1.5 | 1.2 | 1.0 | 1.0 | 0.8 | 0.9 | 1.0 |
| Total expenses | 42.3 | 31.4 | 25.1 | 24.3 | 18.4 | 13.5 | 16.0 | 17.5 |
| Net investment income before excise tax | | | | 20.5 | 13.9 | 12.3 | 17.1 | 14.0 |
| |
| Net realized and unrealized loss | | | | | | | | |
| Net realized (loss) gain on investments | -9.5 | -3.2 | -14.7 | | | | | |
| |
| Net unrealized (depreciation) appreciation on investments | | | | | | | | |
| Net realized and unrealized loss | | | | | | | | |
| Net increase in net assets resulting from operations | 21.2 | 27.8 | 6.4 | | | | | |
| |
| Net investment income per common share (in dollars per share) | $1.46 | $1.41 | $1.18 | | | | | |
| Net increase in net assets resulting from operations per common share | 0.0 | 0.0 | 0.0 | | | | | |
| |
| Distributions declared per share (in dollars per share) | $1.28 | $1.25 | $1.25 | | | | | |
| |
| Weighted average shares outstanding (in shares) | 24.7 | 20.0 | 17.5 | | | | | |