Investment in Medallion Bank and Other Controlled Subsidiaries - Schedule of Comprehensive Income and Other Valuation Adjustments on Other Controlled Subsidiaries (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	[4]	Sep. 30, 2018	[4]	Jun. 30, 2018	[4]	Mar. 31, 2018		Dec. 31, 2017	[4]	Sep. 30, 2017	[4]	Jun. 30, 2017	[4]	Mar. 31, 2017	[4]	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Securities, Available-for-sale [Line Items]
Investment income	[1],[2]																							$ 100,836
Interest expense																						$ 24,816	$ 35,045	28,367	[1],[3]
Net interest income/net investment income		$ 26,587	$ 25,415	$ 23,194	$ 22,321	$ 23,003		$ 24,265		$ 24,719		$ 482	[4]	$ 6,020		$ 5,567		$ 3,787		$ 4,250		71,987	97,517	72,469	[1]
Noninterest income	[1]																							42,006
Operating expenses												1,150														$ 4,211	$ 5,249	$ 6,044
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)	[4]	(2,903)		(2,490)		(1,293)		(435)		(7,156)	2,337	(10,722)	[1],[5]
Income tax provision																						709	341	69		(36,226)
Net income (loss) after taxes/net increase (decrease) on net assets resulting from operations		$ (191)	$ 7,435	$ (6,643)	$ 1,395	$ 9,865		$ (3,846)		$ (13,884)		$ (14,874)	[4]	$ 3,345		$ 619		$ (4,797)		$ 1,111		$ (7,865)	1,996	(22,739)	[1]
Net realized/unrealized losses of Medallion Bank	[1]																							14,675
Net realized/unrealized losses on controlled subsidiaries other than Medallion Bank	[1]																							(11,644)
Net increase in net assets resulting from operations of Medallion Bank and other controlled subsidiaries																							$ (1,762)	$ (25,046)		278
Medallion Bank [Member]
Debt Securities, Available-for-sale [Line Items]
Investment income																										111,281
Interest expense																										13,869
Net interest income/net investment income																										97,412
Noninterest income																										121
Operating expenses	[6]																									26,032
Income (loss) before income taxes/net investment loss before taxes																										71,501
Income tax provision																										15,093
Net income (loss) after taxes/net increase (decrease) on net assets resulting from operations																										56,408
Net realized/unrealized losses of Medallion Bank	[6]																									(51,696)
Unrealized appreciation on Medallion Bank	[7]																									5,482
Net realized/unrealized losses on controlled subsidiaries other than Medallion Bank																										(711)
Net increase in net assets resulting from operations of Medallion Bank and other controlled subsidiaries																										9,483
Medallion Bank [Member] | Medallion Financing Trust I [Member]
Debt Securities, Available-for-sale [Line Items]
Net increase in net assets resulting from operations of Medallion Bank and other controlled subsidiaries																										$ 4,712

[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] Included in interest and investment income is $834, $1,869 and $2,268 of paid in kind interest for the years ended December 31, 2019, 2018, and 2017.

[3] Average borrowings outstanding were $1,138,746, $1,198,124, and $334,022, and the related average borrowing costs were 3.08%, 2.37%, and 4.12% for the years ended December 31, 2019, 2018, and 2017.

[4] 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.

[5] 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.

[6] Excluded from operating expenses and included in net realized/unrealized losses of the Bank were $1,476 of unrealized losses on other assets for 2017

[7] Unrealized appreciation on the Bank reflects the adjustment to the investment carrying amount to reflect the dividends declared to the Company and the US Treasury, and the fair value adjustments to the carrying amount of the Bank