Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2018
Quarterly Financial Information Disclosure [Abstract]
Investment  Income	$ 202,674	$ 184,623	$ 181,431	$ 175,376	$ 169,474	$ 157,339	$ 159,456	$ 172,292	$ 142,880
Investment Income (in dollars per share)	$ 0.51	$ 0.47	$ 0.46	$ 0.45	$ 0.44	$ 0.41	$ 0.41	$ 0.45	$ 0.38
Net Investment Income	$ 99,266	$ 89,969	$ 87,005	$ 85,557	$ 81,369	$ 73,229	$ 73,402	$ 81,561	$ 57,545
Net investment income	$ 99,266	$ 89,969	$ 87,005	$ 85,557	$ 81,369	$ 73,229	$ 73,402	$ 81,561	$ 57,545
Net Investment Income (in dollars per share)	$ 0.25	$ 0.23	$ 0.22	$ 0.22	$ 0.21	$ 0.19	$ 0.19	$ 0.21	$ 0.15	$ 0.88	$ 0.75	$ 0.72	$ 0.85	$ 0.79
Net Realized and  Unrealized (Losses) Gains	$ (191,705)	$ (137,425)	$ 77,291	$ 168,056	$ 130,762	$ 170,457	$ 173,006	$ 224,406	$ 110,201
Net Realized and Unrealized (Losses) Gains (in dollars per share)	$ (0.49)	$ (0.35)	$ 0.20	$ 0.43	$ 0.34	$ 0.44	$ 0.45	$ 0.60	$ 0.30
Net increase (decrease) in net assets resulting from operations applicable to Common Stockholders	$ (105,199)	$ (56,643)	$ 157,157	$ 246,411	$ 209,724	$ 242,421	$ 246,008	$ 305,921	$ 167,746
Net Increase (Decrease) in  Net Assets from Operations Applicable to Common Stockholders (in dollars per share)	$ (0.27)	$ (0.14)	$ 0.40	$ 0.63	$ 0.54	$ 0.62	$ 0.64	$ 0.80	$ 0.45