Main Street Capital Corporation Annual report [Section 13 and 15(d), not S-K Item 405]

Annual report [Section 13 and 15(d), not S-K Item 405]

RELATED PARTY TRANSACTIONS - Narrative (Details)

RELATED PARTY TRANSACTIONS - Narrative (Details)	1 Months Ended						12 Months Ended
RELATED PARTY TRANSACTIONS - Narrative (Details)	Jan. 31, 2025 USD ($) plan shares	Nov. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Mar. 31, 2022 USD ($)	Feb. 28, 2022 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2025 USD ($) shares
MSC Income
Related Party Transaction [Line Items]
Share repurchase program, authorized, amount	$ 65,000,000.0
Related Party \| 2015 Deferred Compensation Plan
Related Party Transaction [Line Items]
Amounts of transaction							$ 32,000,000.0
Related Party \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							$ 19,900,000
Related Party \| 2015 Deferred Compensation Plan \| Common Stock
Related Party Transaction [Line Items]
Deferred compensation (in shares) \| shares							200,320
Related Party \| 2015 Deferred Compensation Plan \| Phantom Stock Units
Related Party Transaction [Line Items]
Amounts of transaction							$ 12,100,000
External Investment Manager \| Related Party
Related Party Transaction [Line Items]
Accounts receivable to MSCC and its subsidiaries							11,400,000
External Investment Manager \| Related Party \| Operating Expenses and Amount Due From Tax Sharing Agreement
Related Party Transaction [Line Items]
Accounts receivable to MSCC and its subsidiaries							8,700,000
External Investment Manager \| Related Party \| Dividends Declared But Not Yet Paid
Related Party Transaction [Line Items]
Accounts receivable to MSCC and its subsidiaries							$ 2,700,000
MSC Income
Related Party Transaction [Line Items]
Share purchase program, authorized, amount	$ 20,000,000.0
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%
MSC Income \| Related Party
Related Party Transaction [Line Items]
Units (in shares) \| shares	289,761						1,642,635
Private Loan Fund \| Related Party \| Investment Management Agreement
Related Party Transaction [Line Items]
Amounts of transaction					$ 15,000,000.0	$ 10,000,000.0
Funded commitments							$ 15,000,000.0
Private Loan Fund \| Related Party \| Secured Revolving Promissory Note
Related Party Transaction [Line Items]
Amounts of transaction				$ 15,000,000.0
Interest rate (as a percent)				3.00%
Unused capacity, commitment fee (as a percent)				0.25%
Amount of debt outstanding							12,000,000.0
Private Loan Fund \| Related Party \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							2,200,000
Private Loan Fund II \| Related Party \| Investment Management Agreement
Related Party Transaction [Line Items]
Amounts of transaction			$ 15,000,000.0
Funded commitments							10,500,000
Unfunded commitments							4,500,000
Percentage of total commitments (as a percent)			20.00%
Private Loan Fund II \| Related Party \| 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%
Private Loan Fund II \| Related Party \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							$ 5,300,000