Main Street Capital Corporation Annual report pursuant to Section 13 and 15(d)

COMMON STOCK (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Aug. 31, 2022	Mar. 31, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Subsidiary, Sale of Stock [Line Items]
Proceeds from public offering of common stock, net of offering costs			$ 265,620	$ 98,889	$ 84,381
At-The-Market Offerings
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)			5,407,382	2,332,795	2,645,778
Proceeds from public offering of common stock, net of offering costs			$ 212,400	$ 99,600	$ 84,900
Sale of stock, consideration received on transaction, net			$ 209,900	$ 98,400	$ 83,800
Sale of stock, shares authorized under agreement (in shares)			43,217
Sale of stock, shares not yet settled (in shares)		15,000,000
Sale of stock, remaining number of shares authorized to be purchased (in shares)			10,462,684
At-The-Market Offerings \| Weighted Average
Subsidiary, Sale of Stock [Line Items]
Sale of stock, price per share (in dollars per share)			$ 39.29	$ 42.71	$ 32.10
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