Consolidated Statement of Changes in Stockholders' Equity and Changes in Net Assets - USD ($) $ in Thousands		Total	Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Common Stock [Member]	Common Stock [Member] Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Capital in Excess of Par [Member]	Capital in Excess of Par [Member] Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Treasury Stock [Member]	Treasury Stock [Member] Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Parent [Member]	Parent [Member] Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Noncontrolling Interest [Member]	Noncontrolling Interest [Member] Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Accumulated Undistributed Net Investment Loss [Member]	Accumulated Undistributed Net Investment Loss [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]	Net Unrealized Appreciation on Investment Net of Tax [Member]	Net Unrealized Appreciation on Investment Net of Tax [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]
Balance at Dec. 31, 2017		$ 287,159		$ 273		$ 273,716		$ (24,919)						$ 287,159
Balance (Investment Company Accounting [Member]) at Dec. 31, 2017																		$ (65,592)		$ 103,681
Balance, shares at Dec. 31, 2017				27,294,327				(2,951,243)
Net increase (decrease) in net assets resulting from operations		(14,874)												(14,874)
Net increase (decrease) in net assets resulting from operations | Investment Company Accounting [Member]																		(38,299)		23,425
Stock-based compensation		152		$ 1		151								152
Issuance of restricted stock, net, shares				95,726
Ending balance at Mar. 31, 2018		272,437	$ 299,502	$ 274	$ 274	273,867	$ 273,867	$ (24,919)	$ (24,919)		$ 23,215	$ 23,215		272,437	$ 272,437		$ 27,065
Ending balance (Investment Company Accounting [Member]) at Mar. 31, 2018																		(103,891)	$ 103,891	127,106	$ (127,106)
Ending balance, shares at Mar. 31, 2018				27,390,053	27,390,053			(2,951,243)	(2,951,243)
Change in Accounting Principle, Type [Extensible List]											mfin:AdoptionOfBankHoldingCompanyAccountingMember
Change in Accounting Principle, Type [Extensible List] | Investment Company Accounting [Member]																			mfin:AdoptionOfBankHoldingCompanyAccountingMember		mfin:AdoptionOfBankHoldingCompanyAccountingMember
Balance at Dec. 31, 2017		287,159		$ 273		273,716		$ (24,919)						287,159
Balance (Investment Company Accounting [Member]) at Dec. 31, 2017																		(65,592)		103,681
Balance, shares at Dec. 31, 2017				27,294,327				(2,951,243)
Net income (loss)	[1]	(22,739)
Ending balance at Dec. 31, 2018		290,204		$ 274		274,292		$ (24,919)		$ 13,043			$ (82)	262,608		$ 27,596
Ending balance, shares at Dec. 31, 2018				27,385,600				(2,951,243)
Balance at Mar. 31, 2018		272,437	$ 299,502	$ 274	$ 274	273,867	$ 273,867	$ (24,919)	$ (24,919)		$ 23,215	$ 23,215		272,437	$ 272,437		$ 27,065
Balance (Investment Company Accounting [Member]) at Mar. 31, 2018																		$ (103,891)	$ 103,891	$ 127,106	$ (127,106)
Balance, shares at Mar. 31, 2018				27,390,053	27,390,053			(2,951,243)	(2,951,243)
Net income (loss)		(7,865)								(10,172)				(10,172)		2,307
Distributions to non-controlling interest		(1,776)														(1,776)
Stock-based compensation		425				425								425
Forfeiture of restricted stock, net, shares				(4,453)
Net change in unrealized gains (losses) on investments, net of tax		(82)											(82)	(82)
Ending balance at Dec. 31, 2018		290,204		$ 274		274,292		$ (24,919)		13,043			(82)	262,608		27,596
Ending balance, shares at Dec. 31, 2018				27,385,600				(2,951,243)
Net income (loss)		1,996								(1,762)				(1,762)		3,758
Non-controlling interest equity raised by Medallion Bank	[2]	42,485														42,485
Distributions to non-controlling interest		(2,519)														(2,519)
Stock-based compensation		1,221		$ 2		1,219								1,221
Issuance of restricted stock, net				$ 0
Issuance of restricted stock, net, shares				216,148
Forfeiture of restricted stock, net				$ 0
Forfeiture of restricted stock, net, shares				(3,946)
Net change in unrealized gains (losses) on investments, net of tax		1,081											1,081	1,081
Ending balance at Dec. 31, 2019		$ 334,468		$ 276		275,511		$ (24,919)		11,281			999	263,148		71,320
Ending balance, shares at Dec. 31, 2019		24,646,559		27,597,802				(2,951,243)
Net income (loss)		$ (26,907)								(34,783)				(34,783)		7,876
Distributions to non-controlling interest		(6,043)														(6,043)
Stock-based compensation		2,030		$ 2		2,028								2,030
Issuance of restricted stock, net				$ 0
Issuance of restricted stock, net, shares				229,408
Forfeiture of restricted stock, net				$ 0
Forfeiture of restricted stock, net, shares				(8,755)
Issuance of restricted stock units, net, shares				10,416
Net change in unrealized gains (losses) on investments, net of tax		1,013											1,013	1,013
Ending balance at Dec. 31, 2020		$ 304,561		$ 278		$ 277,539		$ (24,919)		$ (23,502)			$ 2,012	$ 231,408		$ 73,153
Ending balance, shares at Dec. 31, 2020		24,877,628		27,828,871				(2,951,243)

[1] Balance includes the nine months ended December 31, 2018 under Bank Holding Company Accounting and the three months ended March 31, 2018 under Investment Company Accounting.

[2] Refer to Note 18 for details.