Share Data and Net Assets - Schedule of Changes in Net Assets (Details) - USD ($) $ in Thousands |
3 Months Ended | 9 Months Ended | ||||||
|---|---|---|---|---|---|---|---|---|
Jun. 30, 2025 |
Mar. 31, 2025 |
Dec. 31, 2024 |
Jun. 30, 2024 |
Mar. 31, 2024 |
Dec. 31, 2023 |
Jun. 30, 2025 |
Jun. 30, 2024 |
|
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Beginning of period (in shares) | 82,245,000 | 82,245,000 | ||||||
| Net assets at beginning of period | $ 1,475,113 | $ 1,449,815 | $ 1,487,811 | $ 1,524,099 | $ 1,511,651 | $ 1,515,764 | $ 1,487,811 | $ 1,515,764 |
| Net investment income | 33,481 | 39,055 | 44,302 | 44,575 | 41,367 | 44,189 | 116,838 | 130,131 |
| Net unrealized appreciation (depreciation) | 18,572 | (82,023) | (19,614) | 26,199 | (25,252) | (25,025) | (83,100) | (24,100) |
| Net realized gains (losses) | (13,432) | 6,705 | (17,310) | (69,452) | (6,603) | (8,453) | (24,000) | (84,500) |
| (Provision) benefit for taxes on realized and unrealized gains (losses) | (269) | 14 | (139) | (202) | (175) | (176) | (394) | (553) |
| Distributions to stockholders | (36,996) | (41,400) | (45,235) | (45,180) | (44,715) | (48,897) | $ (123,631) | $ (138,792) |
| Issuance of common stock under dividend reinvestment plan (in shares) | 639,964,000 | 295,484,000 | ||||||
| Issuance of common stock under dividend reinvestment plan | 3,751 | 4,289 | 1,455 | 1,873 | 1,853 | 1,936 | $ 9,495 | $ 5,662 |
| Repurchase of common stock under dividend reinvestment plan | (3,751) | (4,289) | (1,455) | |||||
| Issuance of common stock in connection with the "at the market" offering | 0 | 2,947 | 14,221 | 45,973 | 32,313 | 2,947 | 92,507 | |
| Issuance of common stock in private placement | $ 0 | 100,000 | 0 | $ 100,000 | 0 | |||
| End of period (in shares) | 88,086,000 | 88,086,000 | ||||||
| Net assets at end of period | $ 1,476,469 | $ 1,475,113 | $ 1,449,815 | $ 1,496,133 | $ 1,524,099 | $ 1,511,651 | $ 1,476,469 | $ 1,496,133 |
| Common Stock | ||||||||
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Beginning of period (in shares) | 88,086,000 | 82,245,000 | 82,245,000 | 81,396,000 | 78,965,000 | 77,225,000 | 82,245,000 | 77,225,000 |
| Net assets at beginning of period | $ 881 | $ 822 | $ 822 | $ 814 | $ 790 | $ 772 | $ 822 | $ 772 |
| Issuance of common stock under dividend reinvestment plan (in shares) | 269,000 | 276,000 | 95,000 | 100,000 | 97,000 | 99,000 | ||
| Issuance of common stock under dividend reinvestment plan | $ 3 | $ 3 | $ 1 | $ 1 | $ 1 | $ 1 | ||
| Repurchase of common stock under dividend reinvestment plan (in shares) | (269,000) | (276,000) | (95,000) | |||||
| Repurchase of common stock under dividend reinvestment plan | $ (3) | $ (3) | $ (1) | |||||
| Issuance of common stock in connection with the "at the market" offering (in shares) | 169,000 | 749,000 | 2,334,000 | 1,641,000 | ||||
| Issuance of common stock in connection with the "at the market" offering | $ 2 | $ 7 | $ 23 | $ 17 | ||||
| Issuance of common stock in a private placement (in shares) | 5,672,000 | |||||||
| Issuance of common stock in private placement | $ 57 | |||||||
| End of period (in shares) | 88,086,000 | 88,086,000 | 82,245,000 | 82,245,000 | 81,396,000 | 78,965,000 | 88,086,000 | 82,245,000 |
| Net assets at end of period | $ 881 | $ 881 | $ 822 | $ 822 | $ 814 | $ 790 | $ 881 | $ 822 |
| Additional paid-in-capital | ||||||||
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Net assets at beginning of period | 2,367,337 | 2,264,449 | 2,264,449 | 2,248,363 | 2,200,561 | 2,166,330 | 2,264,449 | 2,166,330 |
| Issuance of common stock under dividend reinvestment plan | 3,748 | 4,286 | 1,454 | 1,872 | 1,852 | 1,935 | ||
| Repurchase of common stock under dividend reinvestment plan | (3,748) | (4,286) | (1,454) | |||||
| Issuance of common stock in connection with the "at the market" offering | 2,945 | 14,214 | 45,950 | 32,296 | ||||
| Issuance of common stock in private placement | 99,943 | |||||||
| Net assets at end of period | 2,367,337 | 2,367,337 | 2,264,449 | 2,264,449 | 2,248,363 | 2,200,561 | 2,367,337 | 2,264,449 |
| Accumulated Overdistributed Earnings | ||||||||
| Investment Company, Net Assets [Roll Forward] | ||||||||
| Net assets at beginning of period | (893,105) | (815,456) | (777,460) | (725,078) | (689,700) | (651,338) | (777,460) | (651,338) |
| Net investment income | 33,481 | 39,055 | 44,302 | 44,575 | 41,367 | 44,189 | ||
| Net unrealized appreciation (depreciation) | 18,572 | (82,023) | (19,614) | 26,199 | (25,252) | (25,025) | ||
| Net realized gains (losses) | (13,432) | 6,705 | (17,310) | (69,452) | (6,603) | (8,453) | ||
| (Provision) benefit for taxes on realized and unrealized gains (losses) | (269) | 14 | (139) | (202) | (175) | (176) | ||
| Distributions to stockholders | (36,996) | (41,400) | (45,235) | (45,180) | (44,715) | (48,897) | ||
| Net assets at end of period | $ (891,749) | $ (893,105) | $ (815,456) | $ (769,138) | $ (725,078) | $ (689,700) | $ (891,749) | $ (769,138) |