Main Street Capital Corporation Quarterly report pursuant to Section 13 or 15(d)

COMMON STOCK (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Subsidiary, Sale of Stock [Line Items]
Proceeds from public offering of common stock, net of offering costs		$ 165,160	$ 194,504
At-The-Market Offerings
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		4,186,856		5,407,382
Proceeds from public offering of common stock, net of offering costs		$ 166,800		$ 212,400
Sale of stock, consideration received on transaction, net		$ 165,100		$ 209,900
Sale of stock, shares authorized under agreement (in shares)	15,000,000
Sale of stock, remaining number of shares authorized to be purchased (in shares)		6,275,828
At-The-Market Offerings \| Weighted Average
Subsidiary, Sale of Stock [Line Items]
Sale of stock, price per share (in dollars per share)		$ 39.84		$ 39.29
Public Equity Offering
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)				1,345,500
Sale of stock, price per share (in dollars per share)				$ 42.85
Sale of stock, consideration received on transaction, net				$ 55,100
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)				175,500