Annual report pursuant to Section 13 and 15(d)


Cover - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2022
Feb. 23, 2023
Jun. 30, 2022
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2022    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Securities Act File Number 001-33723    
Entity Registrant Name Main Street Capital Corporation    
Entity Incorporation, State or Country Code MD    
Entity Tax Identification Number 41-2230745    
Entity Address, Address Line One 1300 Post Oak Boulevard    
Entity Address, Address Line Two 8th Floor    
Entity Address, City or Town Houston    
Entity Address, State or Province TX    
Entity Address, Postal Zip Code 77056    
City Area Code 713    
Local Phone Number 350-6000    
Title of 12(b) Security Common Stock, par value $0.01 per share    
Trading Symbol MAIN    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 2,711.4
Entity Common Stock, Shares Outstanding   79,549,888  
Documents Incorporated by Reference Portions of the registrants’ definitive Proxy Statement for its 2023 Annual Meeting of Stockholders, to be filed with the Securities and Exchange Commission, are incorporated by reference in this Annual Report on Form 10-K in response to Part III.    
Document Fiscal Year Focus 2022    
Entity Central Index Key 0001396440    
Amendment Flag false    
Document Fiscal Period Focus FY