CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Various	Main Street Capital Corporation	Preferred Stock Series A-2 Preferred Stock	Preferred Stock Series B Preferred Stock	Preferred Stock Series C Preferred Stock	Common Stock	Common Stock Series B Preferred Stock	Common Stock Series C Preferred Stock	Treasury Stock	Treasury Stock Various	Treasury Stock Main Street Capital Corporation	Additional Paid-In Capital	Accumulated Deficit
Beginning Balance, Shares (in shares) at Dec. 31, 2016				32	0		36,659,000			204,000
Beginning 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
Issuance of preferred stock (in shares)					2,800
Issuance of preferred stock	2,280												2,280
Preferred stock conversion (in shares)					(2,350)		(8,393,000)	(8,392,857)
Preferred stock conversion	0						$ 1						(1)
Forfeited restricted stock (in shares)							(14,000)
Forfeited restricted stock	0
Issuance of restricted stock (in shares)							123,000
Issuance of stock on vested restricted stock units	0
Preferred stock dividends	(12)												(12)
Other equity issuance costs	(11)												(11)
Purchase of treasury stock (in shares)											42,000	7,712,000
Purchase of treasury stock		$ (12)	$ (2,313)								$ (12)	$ (2,313)
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(7,168)													(7,168)
Stock-based compensation	365												365
Issuance of preferred stock (in shares)						1,750
Issuance of preferred stock	1,527												1,527
Preferred stock conversion (in shares)					(375)	(1,225)	(5,423,000)	(1,339,286)	(4,083,333)
Preferred stock conversion	0
Issuance of restricted stock (in shares)							553,000
Issuance of stock on vested restricted stock units	0
Preferred stock dividends	(12)												(12)
Purchase of treasury stock (in shares)										674,000
Purchase of treasury stock	(144)									$ (144)
Ending Balance, Shares (in shares) at Dec. 31, 2018				32	75	525	51,137,000			1,325,000
Ending Balance, Value at Dec. 31, 2018	$ 6,830			$ 0	$ 0		$ 5			$ (496)			$ 184,994	$ (177,673)