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) - Related Party - USD ($)		1 Months Ended					12 Months Ended
	Dec. 29, 2021	Nov. 30, 2024	Sep. 30, 2023	Mar. 31, 2022	Feb. 28, 2022	Dec. 31, 2020	Dec. 31, 2024	Aug. 01, 2024	May 01, 2024	Jan. 31, 2024	Oct. 31, 2023	Sep. 25, 2023	Aug. 01, 2023	May 01, 2023	May 02, 2022
External Investment Manager
Related Party Transaction [Line Items]
Receivable							$ 10,200,000
MSC Income
Related Party Transaction [Line Items]
Shares acquired (in shares)							1,085,111	125,314	157,629	157,035	237,944		174,271	127,877	47,349
Operating Expenses and Amount Due From Tax Sharing Agreement \| External Investment Manager
Related Party Transaction [Line Items]
Receivable							$ 7,800,000
Dividends Declared But Not Yet Paid \| External Investment Manager
Related Party Transaction [Line Items]
Receivable							2,400,000
Dutch Auction \| MSC Income
Related Party Transaction [Line Items]
Shares acquired (in shares)												57,692
Investment Management Agreement \| Private Loan Fund
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
Investment Management Agreement \| Private Loan Fund II
Related Party Transaction [Line Items]
Amounts of transaction			$ 15,000,000.0
Funded commitments							7,400,000
Unfunded commitments							5,000,000.0
Percentage of total commitments (as a percent)			20.00%
Unsecured Revolving Promissory Note \| Private Loan Fund
Related Party Transaction [Line Items]
Amounts of transaction	$ 85,000,000.0
Interest rate (as a percent)	5.00%
Secured Revolving Promissory Note \| Private Loan Fund
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							1,600,000
Secured Revolving Promissory Note \| Private Loan Fund II
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%
2015 Deferred Compensation Plan
Related Party Transaction [Line Items]
Amounts of transaction							$ 26,600,000
2015 Deferred Compensation Plan \| Common Stock
Related Party Transaction [Line Items]
Deferred compensation (in shares)							187,350
2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							$ 15,600,000
2015 Deferred Compensation Plan \| Phantom Stock Units
Related Party Transaction [Line Items]
Amounts of transaction							11,000,000.0
2015 Deferred Compensation Plan \| Private Loan Fund \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							2,100,000
2015 Deferred Compensation Plan \| Private Loan Fund II \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							$ 4,200,000