Portfolio Investments - Industries and Geographical Locations Comprising Greater than 10% of Portfolio Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2024		Jun. 30, 2024
Concentration Risk [Line Items]
Cost	$ 7,329,338		$ 7,447,174
Fair Value	7,476,641	[1]	7,718,243
California
Concentration Risk [Line Items]
Cost	984,392		970,217
Fair Value	$ 783,356		$ 839,303
Investments at Fair Value | Industry Concentration Risk
Concentration Risk [Line Items]
% of Portfolio	100.00%		100.00%
Investments at Fair Value | Geographic Concentration Risk | California
Concentration Risk [Line Items]
% of Portfolio	10.50%		10.90%
Equity Real Estate Investment Trusts (REITs)
Concentration Risk [Line Items]
Cost	$ 905,709		$ 897,181
Fair Value	$ 1,418,837		$ 1,485,332
Equity Real Estate Investment Trusts (REITs) | Investments at Fair Value | Industry Concentration Risk
Concentration Risk [Line Items]
% of Portfolio	19.00%		19.10%
Health Care Providers & Services
Concentration Risk [Line Items]
Cost	$ 679,077		$ 739,721
Fair Value	$ 682,075		$ 821,921
Health Care Providers & Services | Investments at Fair Value | Industry Concentration Risk
Concentration Risk [Line Items]
% of Portfolio	9.10%		10.60%
Consumer Finance
Concentration Risk [Line Items]
Cost	$ 652,817		$ 623,033
Fair Value	$ 755,072		$ 728,320
Consumer Finance | Investments at Fair Value | Industry Concentration Risk
Concentration Risk [Line Items]
% of Portfolio	10.20%		9.40%
All Other Industries
Concentration Risk [Line Items]
Cost	$ 5,091,735		$ 5,187,239
Fair Value	$ 4,620,657		$ 4,682,670
All Other Industries | Investments at Fair Value | Industry Concentration Risk
Concentration Risk [Line Items]
% of Portfolio	61.70%		60.90%

[1] Fair value is determined by or under the direction of our Board of Directors. Unless otherwise indicated by endnote 45 below, all of our investments are valued using significant unobservable inputs. In accordance with ASC 820, such investments are classified as Level 3 within the fair value hierarchy. See Notes 2 and 3 within the accompanying notes to consolidated financial statements for further discussion.