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Fair Value and Maturity of Debt Outstanding - Schedule of Carrying Values and Estimated Fair Values of Debt Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Jan. 16, 2024	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2021	May 27, 2021	Feb. 19, 2021	Jan. 22, 2021	Mar. 01, 2019	Oct. 01, 2018
Debt Instrument [Line Items]
Principal Outstanding	$ 2,454,992		$ 2,610,216
Net Carrying Value	2,433,911		2,584,848
Fair Value	$ 2,343,171		2,400,816
6.375% 2024 Notes
Debt Instrument [Line Items]
Stated rate	6.375%
3.364% 2026 Notes
Debt Instrument [Line Items]
Stated rate	3.364%
3.437% 2028 Notes
Debt Instrument [Line Items]
Stated rate	3.437%
Public Notes
Debt Instrument [Line Items]
Unamortized Discount & Debt Issuance Costs	$ 12,433		17,103
Prospect Capital InterNotes®
Debt Instrument [Line Items]
Unamortized Discount & Debt Issuance Costs	7,999		6,688
Revolving Credit Facility
Debt Instrument [Line Items]
Unamortized Discount & Debt Issuance Costs	22,975		15,569
Credit Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Principal Outstanding	794,796		1,014,703
Net Carrying Value	794,796		1,014,703
Fair Value	$ 794,796		$ 1,014,703
Effective Interest Rate	2.05%		2.05%
Convertible Notes
Debt Instrument [Line Items]
Principal Outstanding	$ 156,168		$ 156,168
Unamortized Discount & Debt Issuance Costs	649		1,577
Net Carrying Value	155,519		154,591
Fair Value	155,632		154,107
Convertible Notes \| 2025 Notes
Debt Instrument [Line Items]
Stated rate									6.375%
Principal Outstanding	156,168		156,168
Unamortized Discount & Debt Issuance Costs	649		1,577
Net Carrying Value	155,519		154,591
Fair Value	$ 155,632		$ 154,107
Effective Interest Rate	6.63%		6.63%
Unsecured Debt \| 6.375% 2024 Notes
Debt Instrument [Line Items]
Stated rate		6.375%	6.375%	6.375%						6.375%
Principal Outstanding			$ 81,240
Unamortized Discount & Debt Issuance Costs			108
Net Carrying Value			81,132
Fair Value			$ 80,818
Effective Interest Rate			6.57%
Unsecured Debt \| 2026 Notes
Debt Instrument [Line Items]
Stated rate							3.706%	3.706%
Principal Outstanding	$ 400,000		$ 400,000
Unamortized Discount & Debt Issuance Costs	3,263		5,244
Net Carrying Value	396,737		394,756
Fair Value	$ 381,344		$ 354,896
Effective Interest Rate	3.98%		3.98%
Unsecured Debt \| 3.364% 2026 Notes
Debt Instrument [Line Items]
Stated rate	336.40%		3.364%			3.364%
Principal Outstanding	$ 300,000		$ 300,000
Unamortized Discount & Debt Issuance Costs	3,388		4,730
Net Carrying Value	296,612		295,270
Fair Value	$ 275,601		$ 252,282
Effective Interest Rate	3.60%		3.60%
Unsecured Debt \| 3.437% 2028 Notes
Debt Instrument [Line Items]
Stated rate	343.70%		3.437%	3.437%	3.437%
Principal Outstanding	$ 300,000		$ 300,000
Unamortized Discount & Debt Issuance Costs	5,782		7,021
Net Carrying Value	294,218		292,979
Fair Value	$ 256,050		$ 230,472
Effective Interest Rate	3.64%		3.64%
Unsecured Debt \| Public Notes
Debt Instrument [Line Items]
Principal Outstanding	$ 1,000,000		$ 1,081,240
Net Carrying Value	987,567		1,064,137
Fair Value	912,995		918,468
Unsecured Debt \| Prospect Capital InterNotes®
Debt Instrument [Line Items]
Principal Outstanding	504,028		358,105
Unamortized Discount & Debt Issuance Costs	7,999		6,688
Net Carrying Value	496,029		351,417
Fair Value	$ 479,748		$ 313,538
Effective Interest Rate	6.33%		5.77%