Borrowings - Schedule of Debt Securitization (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Accretion of discounts and amortization of premiums on investments	$ (12,878)	$ (9,047)	$ 9,394
Amortization of debt issuance costs	7,380	7,337	10,203
Cash paid for interest expense	138,441	72,458	48,766
Average outstanding balance	3,069,412	2,935,846	2,184,010	$ 2,200,950	$ 1,050,155
2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	26,375	10,542	7,598
Amortization of debt issuance costs	161	421	421
Total interest and other debt financing expenses	26,536	10,963	8,019
Cash paid for interest expense	$ 24,271	$ 8,410	$ 7,712
Average stated interest rate	6.50%	2.60%	1.90%
Average outstanding balance	$ 403,098	$ 408,200	$ 408,200
GCIC 2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	32,923	13,422	9,889
Accretion of discounts on notes issued	544	1,789	1,789
Amortization of debt issuance costs	21	68	47
Total interest and other debt financing expenses	33,488	15,279	11,725
Cash paid for interest expense	$ 30,453	$ 10,757	$ 10,238
Average stated interest rate	6.10%	2.50%	1.80%
Average outstanding balance	$ 537,754	$ 546,500	$ 546,500
2020 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	0	0	4,596
Amortization of debt issuance costs	0	0	1,580
Total interest and other debt financing expenses	0	0	6,176
Cash paid for interest expense	0	0	$ 5,101
Average stated interest rate			2.70%
Average outstanding balance	0	0	$ 170,359
SBA Debentures
Debt Instrument [Line Items]
Stated interest expense	0	0	3,851
Amortization of debt issuance costs	0	0	4,621
Total interest and other debt financing expenses	0	0	8,472
Cash paid for interest expense	0	0	$ 4,396
Average stated interest rate			2.70%
Average outstanding balance	0	0	$ 143,179
MS Credit Facility II
Debt Instrument [Line Items]
Stated interest expense	0	702	3,000
Facility fees	0	529	262
Amortization of debt issuance costs	0	535	493
Total interest and other debt financing expenses	0	1,766	3,755
Cash paid for interest expense	0	$ 1,421	$ 5,034
Average stated interest rate		4.00%	2.70%
Average outstanding balance	0	$ 17,452	$ 112,375
Wells Fargo Credit Facility
Debt Instrument [Line Items]
Stated interest expense	0	0	996
Facility fees	0	0	323
Total interest and other debt financing expenses	0	0	1,319
Cash paid for interest expense	0	0	$ 1,614
Average stated interest rate			2.20%
Average outstanding balance	0	0	$ 45,050
DB Credit Facility
Debt Instrument [Line Items]
Stated interest expense	0	0	73
Facility fees	0	0	14
Total interest and other debt financing expenses	0	0	87
Cash paid for interest expense	0	0	$ 840
Average stated interest rate			2.20%
Average outstanding balance	0	0	$ 3,256
JPM Credit Facility
Debt Instrument [Line Items]
Stated interest expense	41,821	13,876	1,200
Facility fees	2,635	2,501	958
Amortization of debt issuance costs	2,793	1,865	710
Total interest and other debt financing expenses	47,249	18,242	2,868
Cash paid for interest expense	$ 44,667	$ 14,948	$ 1,593
Average stated interest rate	6.20%	2.60%	2.00%
Average outstanding balance	$ 678,560	$ 524,105	$ 59,612
2024 Notes
Debt Instrument [Line Items]
Stated interest expense	16,876	16,745	13,463
Accretion of discounts and amortization of premiums on investments	(1,384)	(1,327)	90
Amortization of debt issuance costs	1,986	1,932	1,583
Total interest and other debt financing expenses	17,478	17,350	15,136
Cash paid for interest expense	$ 16,876	$ 15,188	$ 7,238
Average stated interest rate	3.40%	3.40%	3.40%
Average outstanding balance	$ 500,000	$ 496,164	$ 398,904
2026 Notes
Debt Instrument [Line Items]
Stated interest expense	15,000	14,833	6,028
Accretion of discounts on notes issued	531	520	132
Amortization of debt issuance costs	1,493	1,588	595
Total interest and other debt financing expenses	17,024	16,941	6,755
Cash paid for interest expense	$ 15,000	$ 14,319	$ 5,000
Average stated interest rate	2.50%	2.50%	2.50%
Average outstanding balance	$ 600,000	$ 593,425	$ 240,000
2027 Notes
Debt Instrument [Line Items]
Stated interest expense	7,176	7,176	1,156
Accretion of discounts on notes issued	733	733	118
Amortization of debt issuance costs	926	928	153
Total interest and other debt financing expenses	8,835	8,837	1,427
Cash paid for interest expense	$ 7,174	$ 7,415	$ 0
Average stated interest rate	2.10%	2.10%	2.00%
Average outstanding balance	$ 350,000	$ 350,000	$ 56,575
Adviser Revolver
Debt Instrument [Line Items]
Stated interest expense	0	0	0
Cash paid for interest expense	0	0	0
Average outstanding balance	$ 0	$ 0	$ 0