Consolidated Statements of Financial Condition - USD ($) $ in Thousands	Sep. 30, 2022		Sep. 30, 2021
Assets [Abstract]
Investments, at fair value	$ 5,446,356	[1]	$ 4,894,886	[2]
Cash and cash equivalents	117,290		175,593
Foreign currencies (cost of $7,021 and $5,145, respectively)	6,847		5,497
Restricted cash and cash equivalents	56,416		61,824
Restricted foreign currencies (cost of $0 and $1,442, respectively)	0		1,429
Cash collateral held at broker for forward currency contracts	0		6,960
Interest receivable	20,794		18,261
Receivable from investments sold	0		97
Unrealized appreciation on forward currency contracts	32,333		90
Other assets	1,188		278
Total Assets	5,681,224		5,164,915
Liabilities
Debt	3,093,603		2,569,228
Less unamortized debt issuance costs	17,211		17,850
Debt less unamortized debt issuance costs	3,076,392		2,551,378
Interest payable	20,384		12,516
Management and incentive fees payable	33,430		12,247
Accounts payable and other liabilities	6,293		5,788
Payable for investments purchased	0		294
Accrued trustee fees	225		0
Total Liabilities	3,136,724		2,582,223
Commitments and Contingencies (Note 9)
Net Assets
Preferred stock, par value $0.001 per share, 1,000,000 shares authorized, zero shares issued and outstanding as of September 30, 2022 and September 30, 2021	0		0
Common stock, par value $0.001 per share, 350,000,000 shares authorized, 170,895,670 shares issued and outstanding as of September 30, 2022; 200,000,000 shares authorized, 170,028,584 shares issued and outstanding as of September 30, 2021	171		170
Paid in capital in excess of par	2,676,674		2,664,251
Distributable earnings (losses)	(132,345)		(81,729)
Total Net Assets	2,544,500		2,582,692
Total Liabilities and Total Net Assets	$ 5,681,224		$ 5,164,915
Number of common shares outstanding (in shares)	170,895,670		170,028,584
Net asset value per common share (in dollars per share)	$ 14.89		$ 15.19
Non-controlled/non-affiliate company investments
Assets [Abstract]
Investments, at fair value	$ 5,374,594	[1]	$ 4,815,270	[2]
Non-controlled affiliate company investments
Assets [Abstract]
Investments, at fair value	57,689	[1],[3]	61,379	[4],[5]
Controlled affiliate company investments
Assets [Abstract]
Investments, at fair value	$ 14,073	[1],[6]	$ 18,237	[2],[5]

[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.

[3] As defined in the 1940 Act, the Company is deemed to be an “affiliated person” of the portfolio company as the Company owns five percent or more of the portfolio company's voting securities (“non-controlled affiliate”). Transactions related to investments in non-controlled affiliates for the year ended September 30, 2022 were as follows: Portfolio Company Fair value as of September 30, 2021 Gross Additions(a) Gross Reductions(b) Net change in unrealized gain (loss)Net realized gain (loss)Fair value as of September 30, 2022Interest, dividend and fee incomeAbita Brewing Co. LLC$10,050 $413 $(212)$(2,180)$— $8,071 $823  Benetech, Inc. 2,399 2,821 (2,512)(968)— 1,740 227 Elite Dental Partners LLC16,952 4,151 (60)(354)— 20,689 1,313 Paradigm DKD Group, LLC2,627 357 (3,605)(571)1,192 — 2,026 Rubio's Restaurants, Inc17,559 4 (44)(4,345)— 13,174 1,379 Sloan Company, Inc.5,162 1,520 (648)284 — 6,318 127  Switchfly LLC 6,168 1,321 — 208 — 7,697 603  Uinta Brewing Company 462 22 (498)1,040 (1,026)— 6  Total Non-Controlled Affiliates $61,379 $10,609 $(7,579)$(6,886)$166 $57,689 $6,504  (a) Gross additions may include increases in the cost basis of investments resulting from new investments, amounts related to payment-in-kind (“PIK”) interest capitalized and added to the principal balance of the respective loans, the accretion of discounts, the exchange of one or more existing investments for one or more new investments and the movement of an existing portfolio company into this affiliated category from a different category. (b) Gross reductions may include decreases in the cost basis of investments resulting from principal collections related to investment repayments and sales, the amortization of premiums, the reversal of capitalized PIK for non-accrual positions and the exchange of one or more existing securities for one or more new securities.

[4] Transactions related to investments in non-controlled affiliates for the year ended September 30, 2021 were as follows: Portfolio Company Fair value as of September 30, 2020 Gross Additions(a) Gross Reductions(b) Net change in unrealized gain (loss)Net realized gain (loss)Fair value as of September 30, 2021Interest, dividend and fee income Abita Brewing Co. LLC(c) $— $27,863 $(20,062)$2,249 $— $10,050 $931  Benetech, Inc. 2,672 410 (795)112 — 2,399 349  Dental Holdings Corporation 9,320 561 (13,657)1,792 1,984 — 462 Elite Dental Partners LLC15,368 668 (75)991 — 16,952 955 Paradigm DKD Group, LLC2,460 1,196 (1,215)186 — 2,627 19  Rubio's Restaurants, Inc(d) — 28,760 (16,470)11,008 (5,739)17,559 1,792  Sloan Company, Inc., The 4,365 637 (574)900 (166)5,162 67  Switchfly LLC 7,229 453 — (1,514)— 6,168 469  Uinta Brewing Company 586 266 (209)(181)— 462 (3) Total Non-Controlled Affiliates $42,000 $60,814 $(53,057)$15,543 $(3,921)$61,379 $5,041 

[5] Transactions related to investments in controlled affiliates for the year ended September 30, 2021 were as follows: Portfolio Company Fair value as of September 30, 2020 Gross Additions(a) Gross Reductions(b) Net change in unrealized gain (loss)Net realized gain (loss)Fair value as of September 30, 2021Interest, dividend and fee incomeMMan Acquisition Co.$18,736 $5,023 $(5,023)$(499)$— $18,237 $(12) Total Controlled Affiliates $18,736 $5,023 $(5,023)$(499)$— $18,237 $(12)

[6] As defined in the 1940 Act, the Company is deemed to be both an “affiliated person” of and “control” this portfolio company as the Company owns more than 25% of the portfolio company's outstanding voting securities or has the power to exercise control over management or policies of such portfolio company (including through a management agreement) (“controlled affiliate”). Transactions related to investments in controlled affiliates for the year ended September 30, 2022 were as follows: Portfolio Company Fair value as of September 30, 2021 Gross Additions(a) Gross Reductions(b) Net change in unrealized gain (loss)Net realized gain (loss)Fair value as of September 30, 2022Interest, dividend and fee incomeMMan Acquisition Co.$18,237 $4,956 $(2,996)$(6,124)$— $14,073 $102  Total Controlled Affiliates $18,237 $4,956 $(2,996)$(6,124)$— $14,073 $102  (a) Gross additions may include increases in the cost basis of investments resulting from new investments, amounts related to PIK interest capitalized and added to the principal balance of the respective loans, the accretion of discounts, the exchange of one or more existing investments for one or more new investments and the movement of an existing portfolio company into this affiliated category from a different category. (b) Gross reductions may include decreases in the cost basis of investments resulting from principal collections related to investment repayments or sales, the amortization of premiums, the reversal of capitalized PIK for non-accrual positions and the exchange of one or more existing securities for one or more new securities.