Main Street Capital Corporation Quarterly report [Sections 13 or 15(d)]

Quarterly report [Sections 13 or 15(d)]

RELATED PARTY TRANSACTIONS - Narrative (Details)

RELATED PARTY TRANSACTIONS - Narrative (Details)	1 Months Ended						3 Months Ended
RELATED PARTY TRANSACTIONS - Narrative (Details)	Jan. 31, 2025 USD ($) plan	Nov. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Mar. 31, 2022 USD ($)	Feb. 28, 2022 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2025 USD ($) shares	Jan. 30, 2025 shares	Aug. 01, 2024 shares	May 01, 2024 shares	Jan. 31, 2024 shares	Oct. 31, 2023 shares	Aug. 01, 2023 shares	May 01, 2023 shares	May 02, 2022 shares
MSC Income
Related Party Transaction [Line Items]
Share repurchase program, authorized, amount	$ 65,000,000
MSC Income
Related Party Transaction [Line Items]
Share purchase program, authorized, amount	$ 20,000,000
Share purchase program, period in force	12 months
Share purchase (repurchase) program, number of active plans \| plan	2
Share purchase program, percentage of outstanding stock maximum (as a percent)	3.00%
Related Party \| 2015 Deferred Compensation Plan
Related Party Transaction [Line Items]
Amounts of transaction							$ 27,900,000
Related Party \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							$ 17,500,000
Related Party \| 2015 Deferred Compensation Plan \| Common Stock
Related Party Transaction [Line Items]
Deferred compensation (in shares) \| shares							183,129
Related Party \| 2015 Deferred Compensation Plan \| Phantom Stock Units
Related Party Transaction [Line Items]
Amounts of transaction							$ 10,400,000
Related Party \| External Investment Manager
Related Party Transaction [Line Items]
Receivable							9,200,000
Related Party \| External Investment Manager \| Operating Expenses and Amount Due From Tax Sharing Agreement
Related Party Transaction [Line Items]
Receivable							6,700,000
Related Party \| External Investment Manager \| Dividends Declared But Not Yet Paid
Related Party Transaction [Line Items]
Receivable							$ 2,500,000
Related Party \| MSC Income
Related Party Transaction [Line Items]
Shares acquired (in shares) \| shares							1,374,872	289,761	125,314	157,629	157,035	237,944	174,271	127,877	47,349
Related Party \| Private Loan Fund I \| Investment Management Agreement
Related Party Transaction [Line Items]
Amounts of transaction					$ 15,000,000.0	$ 10,000,000.0
Funded commitments							$ 14,200,000
Unfunded commitments							800,000
Related Party \| Private Loan Fund I \| Secured Revolving Promissory Note
Related Party Transaction [Line Items]
Amounts of transaction				$ 10,000,000.0
Interest rate (as a percent)				5.00%
Unused capacity, commitment fee (as a percent)				0.25%
Amount of debt outstanding							10,000,000.0
Related Party \| Private Loan Fund I \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							2,100,000
Related Party \| Private Loan Fund II \| Investment Management Agreement
Related Party Transaction [Line Items]
Amounts of transaction			$ 15,000,000.0
Funded commitments							8,200,000
Unfunded commitments							5,400,000
Percentage of total commitments (as a percent)			20.00%
Related Party \| Private Loan Fund II \| Secured Revolving Promissory Note
Related Party Transaction [Line Items]
Amounts of transaction		$ 10,000,000.0	$ 50,000,000.0
Interest rate (as a percent)		3.00%	3.50%
Unused capacity, commitment fee (as a percent)		0.25%	0.25%
Amount of debt outstanding							0
Interest rate, floor (as a percent)		2.00%	2.00%
Related Party \| Private Loan Fund II \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							$ 4,200,000