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	3 Months Ended		12 Months Ended
	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Subsidiary, Sale of Stock [Line Items]
Proceeds from public offering of common stock, net of offering costs		$ 40,896	$ 63,522
At-The-Market Offerings
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		1,055,416		5,407,382
Proceeds from public offering of common stock, net of offering costs		$ 41,300		$ 212,400
Sale of stock, consideration received on transaction, net		$ 40,900		$ 209,900
Sale of stock, shares not yet settled (in shares)		27,570
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)		9,407,268
At-The-Market Offerings \| Weighted Average
Subsidiary, Sale of Stock [Line Items]
Sale of stock, price per share (in dollars per share)		$ 39.15		$ 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