CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Main Street Capital Corporation	Various	Preferred Stock Series A-2 Preferred Stock	Preferred Stock Series B Preferred Stock	Common Stock	Treasury Stock	Treasury Stock Main Street Capital Corporation	Treasury Stock Various	Additional Paid-In Capital	Accumulated Deficit
Beginning balance, Shares (in shares) at Dec. 31, 2015				32	0	35,889,000	179,000
Beginning Balance, Value at Dec. 31, 2015	$ 6,383			$ 0	$ 0	$ 4	$ (206)			$ 179,342	$ (172,757)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(3,533)										(3,533)
Stock-based compensation	836									836
Equity issuance costs	(30)									(30)
Issuance of restricted stock (in shares)						170,000
Issuance of restricted stock	93									93
Issuance of stock (in shares)						600,000
Issuance of stock	204									204
Preferred stock dividends	(12)									(12)
Repurchase of common stock, Shares (in shares)							25,000
Repurchase of common stock	(13)						$ (13)
Ending balance, Shares (in shares) at Dec. 31, 2016				32	0	36,659,000	204,000
Ending Balance, Value at Dec. 31, 2016	3,928			$ 0	$ 0	$ 4	$ (219)			180,433	(176,290)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	5,785										5,785
Stock-based compensation	458									458
Equity issuance costs	(11)									(11)
Issuance of restricted stock (in shares)						123,000
Issuance of restricted stock	0
Issuance of stock (in shares)					2,800
Issuance of stock	2,280									2,280
Preferred stock conversion, Shares (in shares)					(2,350)	8,392,857
Preferred stock conversion	0					$ 1				(1)
Forfeited restricted stock (in shares)						(14,000)
Forfeited restricted stock	0
Preferred stock dividends	(12)									(12)
Repurchase of common stock, Shares (in shares)								7,712,000	42,000
Repurchase of common stock		$ (2,313)	$ (12)					$ (2,313)	$ (12)
Issuance of shares from treasury for SRS Note Exchange (in shares)							(7,307,000)
Issuance of shares from treasury for SRS Note Exchange	2,159						$ 2,192			(33)
Ending balance, Shares (in shares) at Dec. 31, 2017				32	450	45,161,000	651,000
Ending Balance, Value at Dec. 31, 2017	$ 12,262			$ 0	$ 0	$ 5	$ (352)			$ 183,114	$ (170,505)