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
	Aug. 31, 2022	Mar. 31, 2022	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Subsidiary, Sale of Stock [Line Items]
Proceeds from public offering of common stock, net of offering costs			$ 194,504	$ 44,846
At-The-Market Offerings
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)			3,429,904		2,332,795
Proceeds from public offering of common stock, net of offering costs			$ 140,600		$ 99,600
Sale of stock, consideration received on transaction, net			$ 139,200		$ 98,400
Sale of stock, shares authorized under agreement (in shares)		15,000,000
Sale of stock, shares not yet settled (in shares)			153,023		36,136
Sale of stock, remaining number of shares authorized to be purchased (in shares)			12,440,162
At-The-Market Offerings \| Weighted Average
Subsidiary, Sale of Stock [Line Items]
Sale of stock, price per share (in dollars per share)			$ 40.99		$ 42.71
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