| Total investment income | $218.5M-6.0% | $232.4M+1.4% | $229.3M+29.2% | $177.5M+27.4% | $139.4M |
| Management fees | $31.2M-0.6% | $31.4M-0.9% | $31.7M | — | — |
| Performance-based incentive fees | $21.7M-9.6% | $24.0M+5.0% | $22.9M | — | — |
| Interest and other credit facility expenses | $67.9M-5.0% | $71.5M-1.4% | $72.5M | — | — |
| Administrative services expense | $5.8M+5.3% | $5.5M-6.4% | $5.9M | — | — |
| Other general and administrative expenses | $5.0M+29.6% | $3.9M-18.8% | $4.8M-22.0% | $6.1M+38.9% | $4.4M |
| Total expenses | $131.7M-3.4% | $136.3M-1.1% | $137.7M+34.1% | $102.7M+30.9% | $78.4M |
| Performance-based incentive fees waived | -$48K+68.6% | -$153K+69.4% | -$500K | — | — |
| Net expenses | $131.6M-3.3% | $136.1M-0.8% | $137.2M+35.7% | $101.1M+29.0% | $78.4M |
| Net investment income | $86.9M-9.7% | $96.3M+4.6% | $92.1M+20.6% | $76.4M+25.3% | $60.9M |
| Net realized gain (loss) on investments and cash equivalents: | $0 | $0+100.0% | -$381K | — | — |
| Net change in unrealized gain (loss) on investments | $8.9M+224.5% | $2.7M+140.4% | -$6.8M+63.7% | -$18.6M-304.4% | $9.1M |
| Net realized and unrealized gain (loss) on investments and cash equivalents | $5.6M+1112.1% | -$553K+96.5% | -$15.7M | — | — |
| Net increase in net assets resulting from operations | $92.5M-3.4% | $95.8M+25.4% | $76.4M+316.5% | $18.3M-69.2% | $59.6M |
| EARNINGS PER SHARE, Basic | $2K-3.4% | $2K+25.7% | $1K | — | — |
| EARNINGS PER SHARE, Diluted | $2K-3.4% | $2K+25.7% | $1K | — | — |
| Companies less than 5% owned [Member] |
| Interest | $13.0M-0.3% | $13.1M+12.3% | $11.6M+22.2% | $9.5M-16.2% | $11.4M |
| Other income | $444K-13.3% | $512K+128.6% | $224K | $0-100.0% | $24K |
| Net realized gain (loss) on investments and cash equivalents: | $0 | $0+100.0% | -$381K | — | — |
| Net change in unrealized gain (loss) on investments | $8.9M+224.5% | $2.7M+140.4% | -$6.8M+63.7% | -$18.6M-304.4% | $9.1M |
| Companies 5% to 25% owned [Member] |
| Dividends | $67.3M+27.1% | $52.9M+15.1% | $46.0M+3.6% | $44.4M+18.2% | $37.6M |
| Net change in unrealized gain (loss) on investments | $8.9M+224.5% | $2.7M+140.4% | -$6.8M+63.7% | -$18.6M-304.4% | $9.1M |
| Companies 5% to 25% owned [Member] |
| Interest | $13.0M-0.3% | $13.1M+12.3% | $11.6M+22.2% | $9.5M-16.2% | $11.4M |
| Other income | $444K-13.3% | $512K+128.6% | $224K | $0-100.0% | $24K |
| Companies more than 25% owned [Member] |
| Interest | $13.0M-0.3% | $13.1M+12.3% | $11.6M+22.2% | $9.5M-16.2% | $11.4M |
| Dividends | $67.3M+27.1% | $52.9M+15.1% | $46.0M+3.6% | $44.4M+18.2% | $37.6M |
| Other income | $444K-13.3% | $512K+128.6% | $224K | $0-100.0% | $24K |
| Net realized gain (loss) on investments and cash equivalents: | $0 | $0+100.0% | -$381K | — | — |
| Net change in unrealized gain (loss) on investments | $8.9M+224.5% | $2.7M+140.4% | -$6.8M+63.7% | -$18.6M-304.4% | $9.1M |
