Quarterly Results of Operations (Unaudited) - Schedule of Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ 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]	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Net interest income/net investment income	$ 26,587	$ 25,415	$ 23,194	$ 22,321	$ 23,003		$ 24,265		$ 24,719		$ 482		$ 6,020		$ 5,567		$ 3,787		$ 4,250		$ 71,987	$ 97,517	$ 72,469
Income (loss) before income taxes/net investment loss before taxes	2,076	7,600	(8,478)	1,139	14,712		(3,963)		(17,905)		(3,566)		(2,903)		(2,490)		(1,293)		(435)		(7,156)	2,337	(10,722)	[3]
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)
Net income (loss) attributable to Medallion Financial Corp./net decrease in net assets resulting from operations	$ (465)	$ 4,975	$ (7,500)	$ 1,228	$ 9,172		$ (4,697)		$ (14,647)		$ (14,874)												$ (25,046)
Net increase (decrease) in net assets resulting from operations per common share
Basic net income (loss) per share	$ (0.02)	$ 0.20	$ (0.31)	$ 0.05	$ 0.38		$ (0.19)		$ (0.60)		$ (0.62)		$ 0.14		$ 0.03		$ (0.20)		$ 0.05			$ (0.07)	$ (1.03)		$ 0.01
Diluted net income (loss) per share	$ (0.02)	$ 0.20	$ (0.31)	$ 0.05	$ 0.38		$ (0.19)		$ (0.60)		$ (0.62)		$ 0.14		$ 0.03		$ (0.20)		$ 0.05			$ (0.07)	$ (1.03)		$ 0.01

[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.

[3] Includes $256 and $870 of net revenues received from Medallion Bank for the years ended December 31, 2018 and 2017, primarily for expense reimbursements. See Notes 6 and 14 for additional information.