| 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, Net [Abstract] | | | | | | | | |
| Interest and Other Income [Abstract] | | | | | | | | |
| Investment Income, Interest | 116.6 | 82.8 | 88.6 | | | | | |
| Noninterest Income | -43.3 | -16.4 | 9.8 | 1.2 | 1.5 | 1.0 | 0.2 | 0.4 |
| Gross Investment Income, Operating | 119.4 | 87.4 | 91.2 | | | | | |
| |
| Operating Expenses [Abstract] | | | | | | | | |
| Base management fee | 15.8 | 12.5 | 12.4 | | | | | |
| Income incentive fee | 6.7 | 10.3 | 8.7 | | | | | |
| Interest Expense, Debt | 26.8 | 17.4 | 15.5 | | | | | |
| Other Expenses | 2.3 | 2.0 | 2.1 | | | | | |
| General and Administrative Expense | 4.4 | 4.1 | 4.6 | 4.3 | 3.3 | 2.9 | 2.9 | 2.8 |
| Operating Expenses | 55.9 | 46.3 | 43.3 | 4.3 | 3.3 | 25.2 | 2.9 | 20.1 |
| Net Investment Income | 63.6 | 41.1 | 47.9 | 38.3 | 35.0 | 26.3 | 23.0 | 22.0 |
| |
| Realized and Unrealized Gain (Loss), Investment and Derivative, Operating, before Tax [Abstract] | | | | | | | | |
| Realized Investment Gains (Losses) | -46.0 | -20.0 | 8.6 | | | | | |
| |
| Unrealized Gain (Loss) on Investments | | | | | | | | |
| Gain (Loss) on Extinguishment of Debt | | | | | | | | |
| Gain (Loss) on Investments | -46.0 | -20.0 | 8.6 | | | | | |
| |
| Diluted net investment income per share of common stock (in dollars per share) | $1.94 | $1.33 | $1.57 | | | | | |
| Basic net investment income per share of common stock (in dollars per share) | $1.94 | $1.33 | $1.57 | | | | | |
| Net increase (decrease) in net assets resulting from operations per share of common stock | 0.0 | 0.0 | 0.0 | | | | | |
| |
| Basic weighted average shares of common stock outstanding (in shares) | 32.7 | 30.9 | 30.6 | 24.8 | 20.5 | 16.3 | 16.2 | 15.0 |
| Diluted weighted average shares of common stock outstanding (in shares) | 32.7 | 30.9 | 30.6 | 24.8 | 20.5 | 16.3 | 16.2 | 15.0 |
| |
| Regular Distributions Declared Per Share | | | | | | | | |
| Special Distributions Declared, Per Share | | | | | | | | |
| Earnings Per Share, Diluted, Distributed | 0.0 | 0.0 | 0.0 | | | | | |
| Earnings Per Share, Basic, Distributed | 0.0 | 0.0 | 0.0 | | | | | |