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]

DEBT - Notes??Due and SBIC Debentures (Details)

DEBT - Notes Due and SBIC Debentures (Details) - USD ($)	1 Months Ended							12 Months Ended
	Aug. 31, 2025	Sep. 30, 2024	Jun. 30, 2024	May 31, 2024	Jan. 31, 2024	Oct. 31, 2021	Jan. 31, 2021	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Sep. 30, 2025
Debt Instrument [Line Items]
Outstanding Balance								$ 2,468,000,000	$ 2,134,000,000
Debt								2,457,594,000	2,122,371,000
July 2026 Notes \| Unsecured Notes
Debt Instrument [Line Items]
Face amount						$ 200,000,000.0	$ 300,000,000.0	500,000,000	500,000,000
Debt instrument, interest rate, stated (as a percent)							3.00%
Debt instrument, issuance price (as a percent)						101.741%	99.004%
Outstanding Balance								500,000,000	500,000,000
Debt								499,715,000	499,188,000
June 2027 Notes \| Unsecured Notes
Debt Instrument [Line Items]
Face amount			$ 300,000,000.0					$ 400,000,000.0	400,000,000
Debt instrument, interest rate, stated (as a percent)			6.50%					6.34%
Debt instrument, issuance price (as a percent)		102.134%	99.793%
Outstanding Balance								$ 400,000,000	400,000,000
Debt								399,569,000	399,282,000
June 2027 Notes, Additional \| Unsecured Notes
Debt Instrument [Line Items]
Face amount		$ 100,000,000.0
Debt instrument, interest rate, stated (as a percent)		5.617%
August 2028 Notes \| Unsecured Notes
Debt Instrument [Line Items]
Face amount	$ 350,000,000.0							350,000,000
Debt instrument, interest rate, stated (as a percent)	5.40%
Debt instrument, issuance price (as a percent)	99.989%
Outstanding Balance								350,000,000
Debt								347,996,000	0
March 2029 Notes \| Unsecured Notes
Debt Instrument [Line Items]
Face amount					$ 350,000,000.0			350,000,000	350,000,000
Debt instrument, interest rate, stated (as a percent)					6.95%
Debt instrument, issuance price (as a percent)					99.865%
Outstanding Balance								350,000,000	350,000,000
Debt								347,721,000	347,002,000
SBIC debentures
Debt Instrument [Line Items]
Repayments of debt								0	63,800,000	$ 16,000,000
SBIC debentures \| Unsecured Notes
Debt Instrument [Line Items]
Face amount								350,000,000	350,000,000
Outstanding Balance								$ 350,000,000	$ 350,000,000
Debt instrument, term (in years)								10 years
Debt instrument, weighted-average annual interest rate (as a percent)								3.30%	3.30%
Debt instrument, remaining term (in years)								4 years 7 months 6 days
Debt								$ 344,593,000	$ 343,417,000
SBIC debentures \| Unsecured Notes \| MSMF
Debt Instrument [Line Items]
Face amount								175,000,000.0
Debt								170,900,000
Debt instrument, unamortized debt issuance costs								4,100,000
SBIC debentures \| Unsecured Notes \| MSC III
Debt Instrument [Line Items]
Face amount								175,000,000.0
Debt								173,700,000
Debt instrument, unamortized debt issuance costs								1,300,000
SBIC debentures \| Unsecured Notes \| Maximum
Debt Instrument [Line Items]
Outstanding Balance								$ 350,000,000.0
May 2024 Notes \| Unsecured Notes
Debt Instrument [Line Items]
Debt instrument, interest rate, stated (as a percent)				5.20%
Repayments of debt				$ 450,000,000.0
December 2025 Series A Notes \| Unsecured Notes
Debt Instrument [Line Items]
Face amount											$ 100,000,000.0
Debt instrument, interest rate, stated (as a percent)											7.84%
December 2025 Series B Notes \| Unsecured Notes
Debt Instrument [Line Items]
Face amount											$ 50,000,000.0
Debt instrument, interest rate, stated (as a percent)											7.53%