Fair Value and Maturity of Debt Outstanding - Schedule Senior Securities (Details) - USD ($)				3 Months Ended
	Jul. 12, 2021	May 26, 2021	Aug. 03, 2020	Sep. 30, 2022	Sep. 30, 2021	Jun. 30, 2022	Jun. 09, 2022	Jun. 30, 2021	May 27, 2021	Jan. 22, 2021	Oct. 30, 2020	Jun. 30, 2020	Jun. 30, 2019	Dec. 05, 2018	Oct. 01, 2018	Jun. 30, 2018	Jun. 20, 2018	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2013	Mar. 15, 2013
Debt Instrument [Line Items]
Total Amount Outstanding				$ 2,640,020,000		$ 2,737,276,000
Asset Coverage per Unit				$ 2,049,000		$ 2,156,000		$ 2,584,000				$ 2,408,000	$ 2,365,000			$ 2,452,000		$ 2,251,000	$ 2,269,000	$ 2,241,000	$ 2,305,000	$ 2,578,000
Involuntary Liquidating Preference per Unit (usd per share)				$ 25.00		$ 25.00
Preferred stock, liquidation preference (usd per share)				$ 25
Long term debt and temporary equity gross liquidation value				$ 3,691,782,000		$ 3,509,353,000		2,404,689,000				2,169,899,000	2,421,526,000			2,346,563,000		2,681,435,000	2,707,465,000	2,983,736,000	2,773,051,000	1,683,002,000
Unfunded commitment coverage, asset coverage per unit				2,026,000
Financial commitment, future amount				43,434,000		43,934,000
5.50% Preferred Stock
Debt Instrument [Line Items]
Asset Coverage per Unit				51		54		65
Preferred Stock, Gross Liquidation Value				$ 871,607,000		$ 590,197,000		$ 137,040,000
Involuntary Liquidating Preference per Unit (usd per share)				$ 25		$ 25	$ 25.00
Preferred stock, liquidation preference (usd per share)			$ 25.00					$ 25			$ 25.00
Dividend rate	5.50%	5.50%	5.50%	5.50%	5.50%
5.35% Preferred Stock
Debt Instrument [Line Items]
Asset Coverage per Unit				$ 51		$ 54
Preferred Stock, Gross Liquidation Value				$ 150,000,000		$ 150,000,000
Involuntary Liquidating Preference per Unit (usd per share)				$ 25		$ 25
Average Market Value per Unit				$ 16.92		$ 21.08
Dividend rate	5.35%
5.00% 2019 Notes
Debt Instrument [Line Items]
Stated rate				5.00%
6.95% 2022 Notes
Debt Instrument [Line Items]
Stated rate				6.95%
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%
Credit Facility
Debt Instrument [Line Items]
Total Amount Outstanding				$ 799,851,000		$ 839,464,000		$ 356,937,000				237,536,000	167,000,000			37,000,000		0	0	368,700,000	92,000,000	124,000,000
Asset Coverage per Unit				$ 9,459,000		9,015,000		17,408,000				22,000,000	34,298,000			155,503,000		0	0	18,136,000	69,470,000	34,996,000
Senior Notes | 2015 Notes
Debt Instrument [Line Items]
Total Amount Outstanding																				150,000,000	150,000,000	150,000,000
Asset Coverage per Unit																				2,241,000	2,305,000	2,578,000
Senior Notes | 2016 Notes
Debt Instrument [Line Items]
Total Amount Outstanding																			167,500,000	167,500,000	167,500,000	167,500,000
Asset Coverage per Unit																			2,269,000	2,241,000	2,305,000	2,578,000
Senior Notes | 2017 Notes
Debt Instrument [Line Items]
Total Amount Outstanding																		50,734,000	129,500,000	130,000,000	130,000,000	130,000,000
Asset Coverage per Unit																		2,251,000	2,269,000	2,241,000	2,305,000	2,578,000
Senior Notes | 2018 Notes
Debt Instrument [Line Items]
Total Amount Outstanding																		85,419,000	200,000,000	200,000,000	200,000,000	200,000,000
Asset Coverage per Unit																		2,251,000	2,269,000	2,241,000	2,305,000	2,578,000
Senior Notes | 2019 Notes
Debt Instrument [Line Items]
Total Amount Outstanding																101,647,000		200,000,000	200,000,000	200,000,000	200,000,000	200,000,000
Asset Coverage per Unit																2,452,000		2,251,000	2,269,000	2,241,000	2,305,000	2,578,000
Senior Notes | 5.00% 2019 Notes
Debt Instrument [Line Items]
Stated rate				5.00%
Total Amount Outstanding																153,536,000		300,000,000	300,000,000	300,000,000	300,000,000
Asset Coverage per Unit																2,452,000		2,251,000	2,269,000	2,241,000	2,305,000
Senior Notes | 2020 Notes
Debt Instrument [Line Items]
Total Amount Outstanding													224,114,000			392,000,000		392,000,000	392,000,000	392,000,000	400,000,000
Asset Coverage per Unit													2,365,000			2,452,000		2,251,000	2,269,000	2,241,000	2,305,000
Senior Notes | 6.95% 2022 Notes
Debt Instrument [Line Items]
Stated rate				6.95%
Total Amount Outstanding																					100,000,000	100,000,000
Asset Coverage per Unit																					$ 2,305,000	$ 2,578,000
Average Market Value per Unit																					$ 1,038,000	$ 1,036,000
Senior Notes | 2022 Notes
Debt Instrument [Line Items]
Total Amount Outstanding						60,501,000		111,055,000				258,240,000	328,500,000			328,500,000		225,000,000
Asset Coverage per Unit						2,733,000		2,740,000				2,408,000	2,365,000			2,452,000		2,251,000
Senior Notes | 2023 Notes
Debt Instrument [Line Items]
Total Amount Outstanding				$ 283,872,000		284,219,000		284,219,000				319,145,000	318,863,000			318,675,000		248,507,000	248,293,000	248,094,000	$ 247,881,000	$ 247,725,000
Asset Coverage per Unit				$ 2,833,000		2,733,000		2,740,000				$ 2,408,000	$ 2,365,000			$ 2,452,000		$ 2,251,000	$ 2,269,000	2,241,000	2,305,000	2,578,000
Average Market Value per Unit												$ 959,000	$ 1,002,000			$ 1,029,000		$ 1,027,000	$ 951,000
Senior Notes | 2024 Notes
Debt Instrument [Line Items]
Total Amount Outstanding												$ 233,788,000	$ 234,443,000			$ 199,281,000		$ 199,281,000	$ 161,364,000
Asset Coverage per Unit												2,408,000	2,365,000			2,452,000		2,251,000	2,269,000
Senior Notes | 6.375% 2024 Notes
Debt Instrument [Line Items]
Stated rate				6.375%
Total Amount Outstanding				$ 81,240,000		81,240,000		81,389,000				99,780,000	99,726,000
Asset Coverage per Unit				2,833,000		2,733,000		2,740,000				2,408,000	2,365,000
Senior Notes | 2025 Notes
Debt Instrument [Line Items]
Total Amount Outstanding				156,168,000		156,168,000		156,168,000				201,250,000	201,250,000
Asset Coverage per Unit				2,833,000		2,733,000		2,740,000				2,408,000	2,365,000
Senior Notes | 2026 Notes
Debt Instrument [Line Items]
Total Amount Outstanding				400,000,000		400,000,000		400,000,000
Asset Coverage per Unit				2,833,000		2,733,000		2,740,000
Senior Notes | 3.364% 2026 Notes
Debt Instrument [Line Items]
Total Amount Outstanding				300,000,000		300,000,000		300,000,000
Asset Coverage per Unit				2,833,000		2,733,000		2,740,000
Senior Notes | 3.437% 2028 Notes
Debt Instrument [Line Items]
Total Amount Outstanding				300,000,000		300,000,000
Asset Coverage per Unit				2,833,000		2,733,000
Senior Notes | 2028 Notes
Debt Instrument [Line Items]
Total Amount Outstanding												70,761,000	70,761,000			55,000,000
Asset Coverage per Unit												$ 2,408,000	$ 2,365,000			$ 2,452,000
Average Market Value per Unit												$ 950,000	$ 984,000			$ 1,004,000
Senior Notes | 2029 Notes
Debt Instrument [Line Items]
Total Amount Outstanding								69,170,000				$ 69,170,000	$ 69,170,000
Asset Coverage per Unit								$ 2,740,000				$ 2,408,000	$ 2,365,000
Average Market Value per Unit								$ 1,028,000				$ 970,000	$ 983,000
Senior Notes | Prospect Capital InterNotes
Debt Instrument [Line Items]
Total Amount Outstanding				349,044,000		347,564,000		$ 508,711,000				$ 680,229,000	$ 707,699,000			$ 760,924,000		980,494,000	908,808,000	827,442,000	785,670,000	363,777,000
Asset Coverage per Unit				2,833,000		2,733,000		$ 2,740,000				$ 2,408,000	$ 2,365,000			$ 2,452,000		$ 2,251,000	$ 2,269,000	$ 2,241,000	$ 2,305,000	$ 2,578,000
Unsecured Debt | 2023 Notes
Debt Instrument [Line Items]
Stated rate																	5.875%						5.875%
Total Amount Outstanding				$ 283,483,000		$ 283,619,000
Unsecured Debt | 6.375% 2024 Notes
Debt Instrument [Line Items]
Stated rate				637.50%		637.50%									6.375%
Total Amount Outstanding				$ 80,986,000		$ 80,941,000
Unsecured Debt | 2026 Notes
Debt Instrument [Line Items]
Stated rate										3.706%				3.706%
Total Amount Outstanding				$ 393,331,000		$ 392,866,000
Unsecured Debt | 3.364% 2026 Notes
Debt Instrument [Line Items]
Stated rate				336.40%		336.40%			3.364%
Total Amount Outstanding				$ 294,294,000		$ 293,974,000
Unsecured Debt | 3.437% 2028 Notes
Debt Instrument [Line Items]
Stated rate				343.70%	3.437%	343.70%
Total Amount Outstanding				$ 292,075,000		$ 291,778,000
Unsecured Debt | Prospect Capital InterNotes
Debt Instrument [Line Items]
Total Amount Outstanding				$ 342,075,000		$ 340,442,000