Portfolio Investments - Portfolio Composition (Details) - USD ($)
$ in Thousands |
12 Months Ended |
Sep. 30, 2023 |
Sep. 30, 2022 |
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 3,044,119
|
$ 2,617,754
|
| Investments at fair value |
$ 2,892,420
|
$ 2,494,111
|
| Investment owned, percent of net assets |
190.82%
|
200.24%
|
| Northeast |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 1,012,955
|
$ 747,420
|
| Investments at fair value |
$ 945,422
|
$ 696,368
|
| Investment owned, percent of net assets |
62.37%
|
55.90%
|
| International |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 418,595
|
$ 301,242
|
| Investments at fair value |
$ 414,079
|
$ 279,646
|
| Investment owned, percent of net assets |
27.32%
|
22.45%
|
| West |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 393,390
|
$ 358,306
|
| Investments at fair value |
$ 384,055
|
$ 345,251
|
| Investment owned, percent of net assets |
25.34%
|
27.72%
|
| Southeast |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 375,247
|
$ 356,041
|
| Investments at fair value |
$ 354,444
|
$ 344,567
|
| Investment owned, percent of net assets |
23.38%
|
27.66%
|
| Midwest |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 360,506
|
$ 373,236
|
| Investments at fair value |
$ 350,620
|
$ 356,934
|
| Investment owned, percent of net assets |
23.13%
|
28.66%
|
| South |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 202,374
|
$ 168,819
|
| Investments at fair value |
$ 188,541
|
$ 166,230
|
| Investment owned, percent of net assets |
12.44%
|
13.35%
|
| Southwest |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 153,318
|
$ 221,308
|
| Investments at fair value |
$ 130,455
|
$ 214,984
|
| Investment owned, percent of net assets |
8.61%
|
17.26%
|
| Northwest |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 127,734
|
$ 91,382
|
| Investments at fair value |
$ 124,804
|
$ 90,131
|
| Investment owned, percent of net assets |
8.23%
|
7.24%
|
| Investment Owned, At Cost | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
100.00%
|
100.00%
|
| Investment Owned, At Cost | Geographic Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
100.00%
|
100.00%
|
| Investment Owned, At Cost | Geographic Concentration Risk | Northeast |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
33.27%
|
28.55%
|
| Investment Owned, At Cost | Geographic Concentration Risk | International |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
13.75%
|
11.51%
|
| Investment Owned, At Cost | Geographic Concentration Risk | West |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
12.92%
|
13.69%
|
| Investment Owned, At Cost | Geographic Concentration Risk | Southeast |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
12.33%
|
13.60%
|
| Investment Owned, At Cost | Geographic Concentration Risk | Midwest |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
11.84%
|
14.26%
|
| Investment Owned, At Cost | Geographic Concentration Risk | South |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
6.65%
|
6.45%
|
| Investment Owned, At Cost | Geographic Concentration Risk | Southwest |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
5.04%
|
8.45%
|
| Investment Owned, At Cost | Geographic Concentration Risk | Northwest |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
4.20%
|
3.49%
|
| Investment Owned, At Fair Value |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investments at fair value |
$ 2,892,420
|
$ 2,494,111
|
| Investment owned, percent of net assets |
190.82%
|
200.24%
|
| Investment Owned, At Fair Value | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
100.00%
|
100.00%
|
| Investment Owned, At Fair Value | Geographic Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
100.00%
|
100.00%
|
| Investment Owned, At Fair Value | Geographic Concentration Risk | Northeast |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
32.69%
|
27.93%
|
| Investment Owned, At Fair Value | Geographic Concentration Risk | International |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
14.32%
|
11.21%
|
| Investment Owned, At Fair Value | Geographic Concentration Risk | West |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
13.28%
|
13.84%
|
| Investment Owned, At Fair Value | Geographic Concentration Risk | Southeast |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
12.25%
|
13.82%
|
| Investment Owned, At Fair Value | Geographic Concentration Risk | Midwest |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
12.12%
|
14.31%
|
| Investment Owned, At Fair Value | Geographic Concentration Risk | South |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
6.52%
|
6.66%
|
| Investment Owned, At Fair Value | Geographic Concentration Risk | Southwest |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
4.51%
|
8.62%
|
| Investment Owned, At Fair Value | Geographic Concentration Risk | Northwest |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
4.31%
|
3.61%
|
| Senior secured debt |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 2,594,640
|
$ 2,227,245
|
| Investments at fair value |
$ 2,501,385
|
$ 2,166,409
|
| Investment owned, percent of net assets |
165.01%
|
173.93%
|
| Senior secured debt | Investment Owned, At Cost | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
85.24%
|
85.08%
|
| Senior secured debt | Investment Owned, At Fair Value | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
86.47%
|
86.86%
|
| Debt investments in the JVs |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 162,986
|
$ 146,444
|
| Investments at fair value |
$ 162,673
|
$ 146,533
|
| Investment owned, percent of net assets |
10.73%
|
11.77%
|
| Debt investments in the JVs | Investment Owned, At Cost | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
5.35%
|
5.59%
|
| Debt investments in the JVs | Investment Owned, At Fair Value | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
5.62%
|
5.88%
|
| Preferred equity |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 99,597
|
$ 85,300
|
| Investments at fair value |
$ 86,057
|
$ 79,523
|
| Investment owned, percent of net assets |
5.68%
|
6.38%
|
| Preferred equity | Investment Owned, At Cost | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
3.27%
|
3.26%
|
| Preferred equity | Investment Owned, At Fair Value | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
2.98%
|
3.19%
|
| Common equity and warrants |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 72,261
|
$ 42,296
|
| Investments at fair value |
$ 57,710
|
$ 24,011
|
| Investment owned, percent of net assets |
3.81%
|
1.93%
|
| Common equity and warrants | Investment Owned, At Cost | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
2.37%
|
1.62%
|
| Common equity and warrants | Investment Owned, At Fair Value | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
2.00%
|
0.96%
|
| Subordinated debt |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 59,844
|
$ 67,147
|
| Investments at fair value |
$ 55,717
|
$ 56,920
|
| Investment owned, percent of net assets |
3.68%
|
4.57%
|
| Subordinated debt | Investment Owned, At Cost | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
1.97%
|
2.57%
|
| Subordinated debt | Investment Owned, At Fair Value | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
1.93%
|
2.28%
|
| LLC equity interests of the JVs |
|
|
| Schedule of Investments [Line Items] |
|
|
| Investment owned, at cost |
$ 54,791
|
$ 49,322
|
| Investments at fair value |
$ 28,878
|
$ 20,715
|
| Investment owned, percent of net assets |
1.91%
|
1.66%
|
| LLC equity interests of the JVs | Investment Owned, At Cost | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
1.80%
|
1.88%
|
| LLC equity interests of the JVs | Investment Owned, At Fair Value | Investment Type Concentration Risk |
|
|
| Schedule of Investments [Line Items] |
|
|
| Concentration risk |
1.00%
|
0.83%
|