Related Party Agreements and Transactions - Additional Information (Details)							3 Months Ended			48 Months Ended	57 Months Ended
	Nov. 02, 2023 USD ($)	Feb. 21, 2023 USD ($)	Jan. 01, 2023	Dec. 31, 2022	Sep. 01, 2022 USD ($)	Jan. 16, 2019 USD ($)	Mar. 31, 2024 USD ($) Component	Dec. 31, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022	Dec. 31, 2022	Sep. 30, 2023 USD ($)
Related Party Transaction [Line Items]
Performance-based incentive fees							$ 6,038,000		$ 6,196,000
Management fees recognized							4,386,000		4,264,000
Management and performance-based incentive fees payable							10,424,000	$ 10,729,000
Administrative services expense							1,223,000		1,422,000
Administrative service expense reimbursements							168,000		335,000
Investments at fair value							2,352,835,000	2,334,199,000
Investment cost							$ 2,552,527,000	$ 2,538,142,000
Co-Investment Holdings
Related Party Transaction [Line Items]
Investment owned, percent of fair value							78.00%	77.00%
Investments at fair value							$ 1,836,370,000	$ 1,808,017,000
Investment owned percent at cost							72.00%	72.00%
Investment cost							$ 1,840,585,000	$ 1,817,084,000
Administrative Service Expense | Merx Aviation Finance, LLC
Related Party Transaction [Line Items]
Administrative service expense reimbursements							75,000		74,000
Incentive Fee based on Income | Legacy Fee
Related Party Transaction [Line Items]
Catch-up amount multiplying percentage			2.1875%
Merx Administration Agreement | Interest Expense
Related Party Transaction [Line Items]
Administrative service expense reimbursements							93,000		261,000
Management services annual fee | MFIC Bethesda CLO 1 LLC Debt Securitization
Related Party Transaction [Line Items]
Management services annual fee	$ 100,000						0	0
AIM
Related Party Transaction [Line Items]
Performance-based incentive fees							6,038,000		6,196,000
Management fees recognized							4,386,000		4,264,000
Management fees waived							$ 0		$ 0
AIM | Base Management Fee
Related Party Transaction [Line Items]
Management fee annual rate percentage							1.75%
Management fee annual rate percentage											1.50%
Management fee quarter rate percentage							0.4375%				0.375%
Management fee annual rate percentage of average gross assets											1.00%
Management fee quarter rate percentage of average gross assets											0.25%
Average gross assets minimum product percentage											200.00%
AIM | Base Management Fee | Maximum
Related Party Transaction [Line Items]
Management fee annual rate percentage									1.50%
Management fee quarter rate percentage									0.375%
AIM | Performance-based Incentive Fee
Related Party Transaction [Line Items]
Number of components consists in incentive fee | Component							2
AIM | Incentive Fee based on Income
Related Party Transaction [Line Items]
Preferred return multiplying net asset percentage			1.75%							1.75%
Preferred return multiplying net assets percentage			1.75%
Percentage of pre-incentive fee net investment income			100.00%							100.00%
Catch-up amount multiplying percentage										2.1875%
Pre-incentive fee net investment income description			The Company will pay the Investment Adviser an incentive fee with respect to our pre-incentive fee net investment income in each calendar quarter as follows:(1) no incentive fee in any calendar quarter in which our pre-incentive fee net investment income for the Trailing Twelve Quarters does not exceed the preferred return amount.(2) 100% of our pre-incentive fee net investment income for the Trailing Twelve Quarters, if any, that exceeds the preferred return amount but is less than or equal to the catch-up amount, which shall be the sum of (i) the product of 2.1875% multiplied by the Company's net asset value at the beginning of each applicable Legacy Fee Quarter included in the relevant Trailing Twelve Quarters and (ii) the product of 2.1212% multiplied by the Company's net asset value at the beginning of each applicable Current Fee Quarter included in the relevant Trailing Twelve Quarters.(3) for any quarter in which the Company’s pre-incentive fee net investment income for the Trailing Twelve Quarters exceeds the catch-up amount, the incentive fee shall equal 20.00% for each Legacy Fee Quarter and 17.50% otherwise of the amount of the Company’s pre-incentive fee net investment income for such Trailing Twelve Quarters, provided, however, that the incentive fee on income for any quarter shall not be greater than 20.00% or 17.50%, as applicable, of the amount of the Company's current quarter’s pre-incentive fee net investment income.				The Company paid the Investment Adviser an incentive fee with respect to our pre-incentive fee net investment income in each calendar quarter as follows: (1) no incentive fee in any calendar quarter in which our pre-incentive fee net investment income for the trailing twelve quarters did not exceed the preferred return amount.(2) 100% of our pre-incentive fee net investment income for the trailing twelve quarters, if any, that exceeded the preferred return amount but is less than or equal to an amount (the “catch-up amount”) determined by multiplying 2.1875% by the Company’s net asset value at the beginning of each applicable calendar quarter comprising the relevant trailing twelve quarters.(3) for any quarter in which the Company’s pre-incentive fee net investment income for the trailing twelve quarters exceeded the catch-up amount, the incentive fee should equal 20% of the amount of the Company’s pre-incentive fee net investment income for such trailing twelve quarters.
Percentage of cumulative pre-Incentive fee net return										20.00%
Cumulative pre-Incentive fee net return description										“Cumulative Pre-Incentive Fee Net Return” during the relevant trailing twelve quarters means (x) Pre-Incentive Fee Net Investment Income in respect of the trailing twelve quarters less (y) any Net Capital Loss, since April 1, 2018, in respect of the trailing twelve quarters. If, in any quarter, the Incentive Fee Cap was zero or a negative value, the Company shall pay no Incentive Fee on Income to the Investment Adviser in that quarter.
AIM | Incentive Fee based on Income | Legacy Fee
Related Party Transaction [Line Items]
Percentage of cumulative pre-Incentive fee net return			20.00%
AIM | Incentive Fee based on Income | Current Fee
Related Party Transaction [Line Items]
Catch-up amount multiplying percentage			2.1212%
Percentage of cumulative pre-Incentive fee net return			17.50%
AIM | Incentive Fee based on Cumulative Net Realized Gains
Related Party Transaction [Line Items]
Percentage of cumulative pre-Incentive fee net return				20.00%			17.50%
Capital gains incentive fee							$ 0		$ 0
Incentive fees waived							0		0
Management and performance-based incentive fees payable							$ 10,424,000	10,729,000
AIM | Fee Offset Agreement
Related Party Transaction [Line Items]
Percentage of management fee revenue earned and incentive fee revenue offset						20.00%
Fee offset step down percentage						10.00%
Fee offset period						7 years
Fee offset period description							The fee offset was supposed to be in place for seven years, however the incentive fees realized by AIM and its affiliates after this seven-year period from applicable investments that were raised or made within the seven-year period would also be used to offset incentive fees payable to AIM by the Company.
Management fee offset							$ 0		274,000
AIM | Fee Offset Agreement | Minimum
Related Party Transaction [Line Items]
Aggregate capital of New Aircraft Funds and New Balance Sheet Investments						$ 3,000,000,000
AIA | Administration Agreement
Related Party Transaction [Line Items]
Administrative services expense							1,223,000		$ 1,422,000
Other Liabilities							$ 0	$ 0
Other Liability, Related Party, Type [Extensible Enumeration]							us-gaap:RelatedPartyMember	us-gaap:RelatedPartyMember
Merx Aviation | First Lien Secured Revolver
Related Party Transaction [Line Items]
Commitment of Merx revolver												$ 100,000,000
Balance of debt amount							$ 70,075,000	$ 74,075,000
Merx Aviation | First Lien Secured Revolver | Common Equity
Related Party Transaction [Line Items]
Debt conversion, converted into common equity					$ 110,700,000
Merx Aviation | Fee Offset Agreement
Related Party Transaction [Line Items]
Agreement termination fee		$ 7,500,000
Merx and Apollo | Fee Offset Agreement
Related Party Transaction [Line Items]
Agreement termination fee		$ 7,500,000