Parent Company Only Condensed Financial Information - Condensed Statements of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended																				9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	[2]
Condensed Statement of Income Captions [Line Items]
Net income (loss)	$ (191)	$ 7,435	$ (6,643)	$ 1,395	$ 9,865		$ (3,846)		$ (13,884)		$ (14,874)		$ 3,345		$ 619		$ (4,797)		$ 1,111		$ (7,865)	$ 1,996	$ (22,739)
Other comprehensive income (loss)																						1,081	(82)
Total comprehensive income (loss) attributable to Medallion Financial Corp.																						(681)	$ (25,128)
Parent Company [Member]
Condensed Statement of Income Captions [Line Items]
Net income (loss)																					(10,172)	(1,762)
Other comprehensive income (loss)																					(82)	1,081
Total comprehensive income (loss) attributable to Medallion Financial Corp.																					$ (10,254)	$ (681)

[1] The three months ended March 31, 2018 and earlier quarters have been accounted for under Investment Company Accounting, and subsequent quarters have been accounted for under Bank Holding Company Accounting.

[2] 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.