Borrowings - Debt Securitizations Narrative (Details) $ in Thousands					12 Months Ended
	Dec. 21, 2020 USD ($)	Aug. 26, 2020 USD ($)	Nov. 16, 2018 USD ($)	Mar. 23, 2018 USD ($)	Sep. 30, 2022 USD ($) portfolioCompany		Sep. 30, 2021 USD ($) portfolioCompany		Sep. 16, 2019 USD ($)	Dec. 13, 2018 USD ($)	Jun. 05, 2014 USD ($)
Debt Instrument [Line Items]
Investments, at fair value					$ 5,446,356	[1]	$ 4,894,886	[2]
2014 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount											$ 402,569
2014 Debt Securitization | Aaa/AAA Class A-1 2014 Note
Debt Instrument [Line Items]
Debt instrument, face amount											191,000
2014 Debt Securitization | Aaa/AAA Class A-2 2014 Note
Debt Instrument [Line Items]
Debt instrument, face amount											20,000
2014 Debt Securitization | Aa2/AA Class B 2014 Notes
Debt Instrument [Line Items]
Debt instrument, face amount											35,000
2014 Debt Securitization | Class C 2014 Notes
Debt Instrument [Line Items]
Debt instrument, received and retained, amount											37,500
Debt instrument, equity interest, amount											$ 119,069
Debt instrument, repurchased face amount				$ 37,500
2014 Debt Securitization | Class C 2014 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				3.50%
2014 Debt Securitization | Class A-1 2014 Notes
Debt Instrument [Line Items]
Debt instrument, repurchased face amount				$ 191,000
2014 Debt Securitization | Class A-1 2014 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				1.75%
2014 Debt Securitization | Class A-1-R 2014 Notes
Debt Instrument [Line Items]
Debt instrument, issued, principal				$ 191,000
2014 Debt Securitization | Class A-1-R 2014 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				0.95%
2014 Debt Securitization | Class A-2 2014 Notes
Debt Instrument [Line Items]
Debt instrument, repurchased face amount				$ 20,000
2014 Debt Securitization | Class A-2 2014 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				1.95%
2014 Debt Securitization | Class A-2-R 2014 Notes
Debt Instrument [Line Items]
Debt instrument, issued, principal				$ 20,000
2014 Debt Securitization | Class A-2-R 2014 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				0.95%
2014 Debt Securitization | Class B 2014 Notes
Debt Instrument [Line Items]
Debt instrument, repurchased face amount				$ 35,000
2014 Debt Securitization | Class B 2014 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				2.50%
2014 Debt Securitization | Class BR 2014 Notes
Debt Instrument [Line Items]
Debt instrument, issued, principal				$ 35,000
2014 Debt Securitization | Class BR 2014 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				1.40%
2014 Debt Securitization | Class C-R 2014 Notes
Debt Instrument [Line Items]
Debt instrument, issued, principal				$ 37,500
2014 Debt Securitization | Class C-R 2014 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				1.55%
2018 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount			$ 602,400
Number of investment portfolio company | portfolioCompany					74		75
Investments, at fair value					$ 568,310		$ 579,075
2018 Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount			113,400
2018 Debt Securitization | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate					2.70%
2018 Debt Securitization | AAA/AAA Class A 2018 Notes | Private Placement
Debt Instrument [Line Items]
Sale of stock, consideration received on transaction			$ 327,000
2018 Debt Securitization | AAA/AAA Class A 2018 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			1.48%
2018 Debt Securitization | AA Class B 2018 Notes | Private Placement
Debt Instrument [Line Items]
Sale of stock, consideration received on transaction			$ 61,200
2018 Debt Securitization | AA Class B 2018 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			2.10%
2018 Debt Securitization | A Class C-1 2018 Notes | Private Placement
Debt Instrument [Line Items]
Sale of stock, consideration received on transaction			$ 20,000
2018 Debt Securitization | A Class C-1 2018 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			2.80%
2018 Debt Securitization | A Class C-2 2018 Notes | Private Placement
Debt Instrument [Line Items]
Sale of stock, consideration received on transaction			$ 38,800
2018 Debt Securitization | A Class C-2 2018 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			2.65%
2018 Debt Securitization | BBB- Class D 2018 Notes | Private Placement
Debt Instrument [Line Items]
Sale of stock, consideration received on transaction			$ 42,000
2018 Debt Securitization | BBB- Class D 2018 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			2.95%
2018 GCIC Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount									$ 908,195
Number of investment portfolio company | portfolioCompany					91		96
Investments, at fair value					$ 885,171		$ 889,326
2018 GCIC Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount										$ 179,695
2018 GCIC Debt Securitization | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate					2.70%
2018 GCIC Debt Securitization | AAA/AAA Class A-1 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount										490,000
2018 GCIC Debt Securitization | AAA Class A-2 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount										38,500
2018 GCIC Debt Securitization | AA Class B-1 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount										18,000
2018 GCIC Debt Securitization | Class B-2 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, received and retained, amount										27,000
2018 GCIC Debt Securitization | Class C GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, received and retained, amount										95,000
2018 GCIC Debt Securitization | Class D GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, received and retained, amount										$ 60,000
2018 GCIC Debt Securitization | Class A-2 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, repurchased face amount	$ 38,500
Debt instrument, basis spread on variable rate	4.665%
2018 GCIC Debt Securitization | Class A-2-R GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, issued, principal	$ 38,500
Debt instrument, basis spread on variable rate	2.498%
2020 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount		$ 330,355
Debt instrument, sinking fund payment		297,355
2020 Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount		108,355
2020 Debt Securitization | AAA Class A-1 2020 Notes
Debt Instrument [Line Items]
Debt instrument, face amount		$ 137,500
2020 Debt Securitization | AAA Class A-1 2020 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.35%
2020 Debt Securitization | AAA Class A-2 2020 Notes
Debt Instrument [Line Items]
Debt instrument, face amount		$ 10,500
2020 Debt Securitization | AAA Class A-2 2020 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.75%
2020 Debt Securitization | AA Class B 2020 Notes
Debt Instrument [Line Items]
Debt instrument, face amount		$ 21,000
2020 Debt Securitization | AA Class B 2020 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		3.20%
2020 Debt Securitization | A Class C 2020 Notes
Debt Instrument [Line Items]
Debt instrument, face amount		$ 33,000
2020 Debt Securitization | A Class C 2020 Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		3.65%
2020 Debt Securitization | AAA Class A-1-L loans | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.35%
Line of credit facility, commitment fee amount		$ 20,000

[1] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6. Fair Value Measurements.

[2] The fair values of substantially all investments were valued using significant unobservable inputs. See Note 6. Fair Value Measurements.