Investment in Medallion Bank and Other Controlled Subsidiaries - Schedule of Comprehensive Income and Other Valuation Adjustments on Other Controlled Subsidiaries under Investment Company Accounting (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Investment income	[1]	$ 30,043
Interest expense	[2]	7,722
Net interest income		22,321
Noninterest income		6,863
Operating expenses			$ 1,150
Net investment income before income taxes	[3]	1,139
Income tax benefit		(256)	(640)
Net income after taxes/net decrease on net assets resulting from operations		1,395
Net decrease in net assets resulting from operations of Medallion Bank		$ 1,228	(14,874)
Medallion Bank [Member]
Debt Securities, Available-for-sale [Line Items]
Investment income			26,880
Interest expense			3,615
Net interest income			23,265
Noninterest income			19
Operating expenses			7,158
Net investment income before income taxes			16,126
Income tax benefit			3,321
Net income after taxes/net decrease on net assets resulting from operations			19,447
Net realized/unrealized losses of Medallion Bank			(28,539)
Unrealized appreciation on Medallion Bank	[4]		39,092
Net realized/unrealized losses on controlled subsidiaries other than Medallion Bank			(885)
Net decrease in net assets resulting from operations of Medallion Bank			29,115
Medallion Bank [Member] | Medallion Financing Trust I [Member]
Debt Securities, Available-for-sale [Line Items]
Net decrease in net assets resulting from operations of Medallion Bank			$ (9,092)

[1] Included in interest and investment income is $237 and $491 of paid in kind interest for the three months ended March 31, 2019 and 2018.

[2] Average borrowings outstanding were $1,067,075 and $324,322, and the related average borrowing costs were 2.93% and 4.44% for the three months ended March 31, 2019 and 2018.

[3] Includes $256 of net revenues received from Medallion Bank for the three months ended March 31, 2018, primarily for expense reimbursements. See Notes 6 and 15 for additional information.

[4] Unrealized depreciation on Medallion Bank reflects the adjustment to the investment carrying amount to reflect the dividends declared to the US Treasury, and the fair value adjustments to the carrying amount of Medallion Bank.