Borrowings - Schedule of Debt Securitization (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Debt Instrument [Line Items]
Accretion of discounts and amortization of premiums on investments			$ 454	$ (3,534)
Amortization of debt issuance costs	$ 1,937	$ 1,798	5,362	5,241
Cash paid for interest expense			98,381	46,115
Average outstanding balance			3,071,718	2,860,373
2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	6,947	2,612	19,207	6,362
Amortization of debt issuance costs	9	105	152	315
Total interest and other debt financing expenses	6,956	2,717	19,359	6,677
Cash paid for interest expense	$ 6,577	$ 1,930	$ 17,246	$ 5,624
Annualized average stated interest rate	6.90%	2.60%	6.30%	2.10%
Average outstanding balance	$ 404,387	$ 408,200	$ 406,929	$ 408,200
GCIC 2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	8,670	3,326	24,068	8,144
Accretion of discounts on notes issued	0	446	544	1,338
Amortization of debt issuance costs	0	17	21	51
Total interest and other debt financing expenses	8,670	3,789	24,633	9,533
Cash paid for interest expense	$ 8,261	$ 2,477	$ 21,681	$ 7,216
Annualized average stated interest rate	6.40%	2.40%	5.90%	2.00%
Average outstanding balance	$ 539,674	$ 546,500	$ 544,225	$ 546,500
MS Credit Facility II
Debt Instrument [Line Items]
Stated interest expense	0	232	0	236
Facility fees	0	256	0	444
Amortization of debt issuance costs	0	53	0	159
Total interest and other debt financing expenses	0	541	0	839
Cash paid for interest expense	0	$ 295	0	$ 591
Annualized average stated interest rate		3.30%		3.30%
Average outstanding balance	0	$ 28,539	0	$ 9,513
JPM Credit Facility
Debt Instrument [Line Items]
Stated interest expense	11,195	4,347	29,463	7,822
Facility fees	1,005	529	2,145	1,855
Amortization of debt issuance costs	836	492	1,948	1,366
Total interest and other debt financing expenses	13,036	5,368	33,556	11,043
Cash paid for interest expense	$ 11,704	$ 3,311	$ 31,491	$ 6,850
Annualized average stated interest rate	6.40%	2.60%	5.90%	2.30%
Average outstanding balance	$ 703,448	$ 663,088	$ 670,564	$ 460,080
2024 Notes
Debt Instrument [Line Items]
Stated interest expense	4,219	4,219	12,657	12,526
Accretion of discounts and amortization of premiums on investments	(345)	(345)	(1,035)	(978)
Amortization of debt issuance costs	495	486	1,485	1,437
Total interest and other debt financing expenses	4,369	4,360	13,107	12,985
Cash paid for interest expense	$ 8,438	$ 8,438	$ 16,876	$ 15,188
Annualized average stated interest rate	3.40%	3.40%	3.40%	3.40%
Average outstanding balance	$ 500,000	$ 500,000	$ 500,000	$ 494,872
2026 Notes
Debt Instrument [Line Items]
Stated interest expense	3,750	3,750	11,250	11,083
Accretion of discounts on notes issued	132	132	397	386
Amortization of debt issuance costs	372	408	1,117	1,202
Total interest and other debt financing expenses	4,254	4,290	12,764	12,671
Cash paid for interest expense	$ 0	$ 0	$ 7,500	$ 6,819
Annualized average stated interest rate	2.50%	2.50%	2.50%	2.50%
Average outstanding balance	$ 600,000	$ 600,000	$ 600,000	$ 591,209
2027 Notes
Debt Instrument [Line Items]
Stated interest expense	1,794	1,794	5,382	5,382
Accretion of discounts on notes issued	183	182	548	548
Amortization of debt issuance costs	225	237	639	711
Total interest and other debt financing expenses	2,202	2,213	6,569	6,641
Cash paid for interest expense	$ 0	$ 0	$ 3,587	$ 3,827
Annualized average stated interest rate	2.10%	2.10%	2.10%	2.10%
Average outstanding balance	$ 350,000	$ 350,000	$ 350,000	$ 350,000
Adviser Revolver
Debt Instrument [Line Items]
Stated interest expense	0	0	0	0
Cash paid for interest expense	0	0	0	0
Average outstanding balance	$ 0	$ 0	$ 0	$ 0