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

Quarterly report [Sections 13 or 15(d)]

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

DEBT - Notes Due and SBIC Debentures (Details) - USD ($)	1 Months Ended							3 Months Ended
	Mar. 31, 2026	Aug. 31, 2025	Sep. 30, 2024	Jun. 30, 2024	Jan. 31, 2024	Oct. 31, 2021	Jan. 31, 2021	Mar. 31, 2026	Dec. 31, 2025	Sep. 30, 2025
Debt Instrument [Line Items]
Debt instrument, outstanding balance	$ 2,536,000,000							$ 2,536,000,000	$ 2,468,000,000
Recorded Value	2,529,576,000							2,529,576,000	2,457,594,000
March 2029 Notes \| Unsecured Notes
Debt Instrument [Line Items]
Face amount	550,000,000				$ 350,000,000.0			550,000,000	350,000,000
Face amount, additional issuance	$ 200,000,000.0							$ 200,000,000.0
Debt instrument, interest rate, stated (as a percent)	6.68%				6.95%			6.68%
Debt Instrument, interest rate, yield-to-worst	6.164%							6.164%
Debt instrument, issuance price (as a percent)	102.061%				99.865%
Debt instrument, outstanding balance	$ 550,000,000							$ 550,000,000	350,000,000
Recorded Value	551,015,000							551,015,000	347,721,000
July 2026 Notes \| Unsecured Notes
Debt Instrument [Line Items]
Face amount	500,000,000					$ 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%
Debt instrument, outstanding balance	500,000,000							500,000,000	500,000,000
Recorded Value	499,846,000							499,846,000	499,715,000
June 2027 Notes \| Unsecured Notes
Debt Instrument [Line Items]
Face amount	$ 400,000,000.0			$ 300,000,000.0				$ 400,000,000.0	400,000,000
Debt instrument, interest rate, stated (as a percent)	6.34%			6.50%				6.34%
Debt instrument, issuance price (as a percent)			102.134%	99.793%
Debt instrument, outstanding balance	$ 400,000,000							$ 400,000,000	400,000,000
Recorded Value	399,641,000							399,641,000	399,569,000
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	$ 350,000,000.0						350,000,000	350,000,000
Debt instrument, interest rate, stated (as a percent)		5.40%
Debt instrument, issuance price (as a percent)		99.989%
Debt instrument, outstanding balance	350,000,000							350,000,000	350,000,000
Recorded Value	348,187,000							348,187,000	347,996,000
SBIC debentures \| Unsecured Notes
Debt Instrument [Line Items]
Face amount	350,000,000							350,000,000	350,000,000
Debt instrument, outstanding balance	$ 350,000,000							$ 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%	3.30%
Debt instrument, remaining term (in years)								4 years 4 months 24 days
Recorded Value	$ 344,887,000							$ 344,887,000	$ 344,593,000
SBIC debentures \| Unsecured Notes \| MSMF
Debt Instrument [Line Items]
Face amount	175,000,000.0							175,000,000.0
Recorded Value	171,100,000							171,100,000
Debt instrument, unamortized debt issuance costs	3,900,000							3,900,000
SBIC debentures \| Unsecured Notes \| MSC III
Debt Instrument [Line Items]
Face amount	175,000,000.0							175,000,000.0
Recorded Value	173,800,000							173,800,000
Debt instrument, unamortized debt issuance costs	1,200,000							1,200,000
SBIC debentures \| Unsecured Notes \| Maximum
Debt Instrument [Line Items]
Debt instrument, outstanding balance	$ 350,000,000.0							$ 350,000,000.0
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%