SHARE-BASED COMPENSATION - Restricted Stock Authorized and Available for Issuance (Details) - Restricted Stock	9 Months Ended
Sep. 30, 2022 shares
Main Street Capital Corporation 2022 Equity and Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Restricted stock authorized under the plan (in shares)	5,000,000
Less net restricted stock granted (in shares)	(10,256)
Restricted stock available for issuance (in shares)	4,989,744
Main Street Capital Corporation 2022 Non-Employee Director Restricted Stock Plan \| Director
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Restricted stock authorized under the plan (in shares)	300,000
Less net restricted stock granted (in shares)	(4,590)
Restricted stock available for issuance (in shares)	295,410

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(01)
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Number of shares authorized for issuance under share-based payment arrangement.

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-Publisher FASB
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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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