Borrowings - Schedule of Debt Securitization (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Accretion of discounts and amortization of premiums on investments	$ (9,047)	$ 9,394	$ 23,483
Amortization of debt issuance costs	7,337	10,203	3,534
Cash paid for interest expense	72,458	48,766	74,933
Average outstanding balance	2,935,846	2,184,010	2,200,950	$ 1,050,155	$ 822,823
2014 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	0	0	2,498
Amortization of debt issuance costs	0	0	0
Total interest and other debt financing expenses	0	0	2,498
Cash paid for interest expense	0	0	$ 3,298
Average stated interest rate			2.80%
Average outstanding balance	0	0	$ 90,526
2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	10,542	7,598	12,616
Amortization of debt issuance costs	421	421	421
Total interest and other debt financing expenses	10,963	8,019	13,037
Cash paid for interest expense	$ 8,410	$ 7,712	$ 14,188
Average stated interest rate	2.60%	1.90%	3.10%
Average outstanding balance	$ 408,200	$ 408,200	$ 408,200
GCIC 2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	13,422	9,889	16,854
Accretion of discounts on notes issued	1,789	1,789	1,355
Amortization of debt issuance costs	68	47	0
Total interest and other debt financing expenses	15,279	11,725	18,209
Cash paid for interest expense	$ 10,757	$ 10,238	$ 19,171
Average stated interest rate	2.50%	1.80%	3.10%
Average outstanding balance	$ 546,500	$ 546,500	$ 546,500
2020 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	0	4,596	505
Amortization of debt issuance costs	0	1,580	74
Total interest and other debt financing expenses	0	6,176	579
Cash paid for interest expense	0	$ 5,101	$ 0
Average stated interest rate		2.70%	2.70%
Average outstanding balance	0	$ 170,359	$ 18,590
SBA Debentures
Debt Instrument [Line Items]
Stated interest expense	0	3,851	9,075
Amortization of debt issuance costs	0	4,621	1,218
Total interest and other debt financing expenses	0	8,472	10,293
Cash paid for interest expense	0	$ 4,396	$ 9,237
Average stated interest rate		2.70%	3.10%
Average outstanding balance	0	$ 143,179	$ 289,003
MS Credit Facility II
Debt Instrument [Line Items]
Stated interest expense	702	3,000	11,018
Facility fees	529	262	551
Amortization of debt issuance costs	535	493	1,821
Total interest and other debt financing expenses	1,766	3,755	13,390
Cash paid for interest expense	$ 1,421	$ 5,034	$ 11,567
Average stated interest rate	4.00%	2.70%	3.10%
Average outstanding balance	$ 17,452	$ 112,375	$ 350,846
WF Credit Facility
Debt Instrument [Line Items]
Stated interest expense	0	996	6,851
Facility fees	0	323	371
Total interest and other debt financing expenses	0	1,319	7,222
Cash paid for interest expense	0	$ 1,614	$ 7,533
Average stated interest rate		2.20%	3.00%
Average outstanding balance	0	$ 45,050	$ 228,100
DB Credit Facility
Debt Instrument [Line Items]
Stated interest expense	0	73	6,554
Facility fees	0	14	586
Total interest and other debt financing expenses	0	87	7,140
Cash paid for interest expense	0	$ 840	$ 8,258
Average stated interest rate		2.20%	3.20%
Average outstanding balance	0	$ 3,256	$ 205,373
JPM Credit Facility
Debt Instrument [Line Items]
Stated interest expense	13,876	1,200	0
Facility fees	2,501	958	0
Amortization of debt issuance costs	1,865	710	0
Total interest and other debt financing expenses	18,242	2,868	0
Cash paid for interest expense	$ 14,948	$ 1,593	0
Average stated interest rate	2.60%	2.00%
Average outstanding balance	$ 524,105	$ 59,612	0
SLF Credit Facility
Debt Instrument [Line Items]
Stated interest expense	0	0	445
Cash paid for interest expense	0	0	$ 445
Average stated interest rate			3.10%
Average outstanding balance	0	0	$ 14,542
GCIC SLF Credit Facility
Debt Instrument [Line Items]
Stated interest expense	0	0	480
Cash paid for interest expense	0	0	$ 487
Average stated interest rate			3.00%
Average outstanding balance	0	0	$ 15,896
2024 Notes
Debt Instrument [Line Items]
Stated interest expense	16,745	13,463	0
Accretion of discounts and amortization of premiums on investments	(1,327)	90	0
Amortization of debt issuance costs	1,932	1,583	0
Total interest and other debt financing expenses	17,350	15,136	0
Cash paid for interest expense	$ 15,188	$ 7,238	0
Average stated interest rate	3.40%	3.40%
Average outstanding balance	$ 496,164	$ 398,904	0
2026 Notes
Debt Instrument [Line Items]
Stated interest expense	14,833	6,028	0
Accretion of discounts on notes issued	520	132	0
Amortization of debt issuance costs	1,588	595	0
Total interest and other debt financing expenses	16,941	6,755	0
Cash paid for interest expense	$ 14,319	$ 5,000	0
Average stated interest rate	2.50%	2.50%
Average outstanding balance	$ 593,425	$ 240,000	0
2027 Notes
Debt Instrument [Line Items]
Stated interest expense	7,176	1,156	0
Accretion of discounts on notes issued	733	118	0
Amortization of debt issuance costs	928	153	0
Total interest and other debt financing expenses	8,837	1,427	0
Cash paid for interest expense	$ 7,415	$ 0	0
Average stated interest rate	2.10%	2.00%
Average outstanding balance	$ 350,000	$ 56,575	0
Adviser Revolver
Debt Instrument [Line Items]
Stated interest expense	0	0	33
Cash paid for interest expense	0	0	$ 42
Average stated interest rate			1.30%
Average outstanding balance	0	0	$ 2,594
Other Short-Term Borrowings
Debt Instrument [Line Items]
Stated interest expense	0	0	1,533
Cash paid for interest expense	0	0	$ 1,533
Average stated interest rate			5.00%
Average outstanding balance	$ 0	$ 0	$ 30,780