| In millions, except per share items | Jun-30-23 | Dec-31-22 | Sep-30-22 | Jun-30-22 | Mar-31-22 | Dec-31-21 | Sep-30-21 | Jun-30-21 |
| | 10-Q | 10-K | 10-Q | 10-Q | 10-Q | 10-K | 10-Q | 10-Q |
| Revenues [Abstract] | | | | | | | | |
| Total interest income | 55.4 | | | 40.8 | | | | |
| Total dividend income | 10.3 | | | 7.2 | | | | |
| Total fee and other income | 4.3 | | | 5.1 | | | | |
| Total payment-in-kind interest income | 5.1 | | | 2.5 | | | | |
| |
| Interest income from cash | | | | | | | | |
| Total investment income | 75.3 | 63.5 | 56.3 | 55.6 | | | | |
| |
| Operating expenses: | | | | | | | | |
| Interest and other financing fees | 20.8 | 16.7 | 15.3 | 13.2 | | | | |
| Base management fee (Note 2) | 8.1 | 8.0 | 8.3 | 7.4 | | | | |
| Incentive management fees (Note 2) | 10.1 | | 1.8 | | | | | |
| General and administrative expenses (Note 2) | 2.4 | 1.2 | 3.0 | 3.3 | 2.5 | 2.3 | 2.3 | 2.2 |
| Total operating expenses | 41.5 | 25.9 | 28.4 | 23.8 | | | | |
| Net investment income before taxes | 33.8 | 37.6 | 27.9 | 31.8 | | | | |
| Income taxes, including excise tax expense | 0.2 | | | | | | | |
| Net investment income after taxes | 33.6 | | | 31.8 | | | | |
| |
| Net realized gains (losses): | | | | | | | | |
| Net realized gains (losses) on investments | -46.2 | | -8.3 | -7.5 | | -0.1 | | |
| Foreign currency transactions | -2.3 | 17.2 | 10.7 | -2.7 | | -6.0 | | |
| Net realized gains (losses) | -48.5 | | | -10.2 | | | | |
| |
| Net unrealized appreciation (depreciation): | | | | | | | | |
| Net unrealized appreciation (depreciation) on investments | 53.7 | | | -61.8 | | | | |
| Credit support agreements | 2.0 | 3.6 | 3.4 | -13.4 | | | | |
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| Foreign currency transactions | | | | | | | | |
| Net unrealized appreciation (depreciation) | | | | | | | | |
| Net realized gains (losses) and unrealized appreciation (depreciation) on investments, credit support agreements and foreign currency transactions | | | | | | | | |
| Benefit from (provision for) income taxes | | | | | | | | |
| Net increase (decrease) in net assets resulting from operations | | | | | | | | |
| Net investment income per share—basic (in dollars per share) | $0.31 | $0.34 | $0.26 | $0.29 | | | | |
| Net investment income per share—diluted (in dollars per share) | $0.31 | $0.34 | $0.26 | $0.29 | | | | |
| Net increase | 0.0 | | | 0.0 | | | | |
| |
| Dividends/distributions per share: | | | | | | | | |
| Total dividends/distributions per share, declared (in dollars per share) | $0.25 | $0.24 | $0.24 | $0.24 | | | | |
| Total dividends/distributions per share, paid (in dollars per share) | $0.25 | $0.24 | $0.24 | $0.24 | | | | |
| |
| Weighted average shares outstanding—basic (in shares) | 107.4 | 102.9 | 109.3 | 110.8 | | | | |
| Weighted average shares outstanding—diluted (in shares) | 107.4 | 102.9 | 109.3 | 110.8 | | | | |