Medallion Bank - Schedule of Comprehensive Income and Other Valuation Adjustments on Other Controlled Subsidiaries (Detail) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Interest expense		$ 7,925
Net interest income		24,719
Operating expenses			$ 1,150	$ 925		$ 2,092
Net investment income before income taxes		(17,905)
Income tax (provision)		(4,021)		(7,018)	$ (4,661)	(8,990)
Net investment income after income taxes		(13,884)
Net decrease in net assets resulting from operations		$ (14,647)		(4,797)	$ (29,521)	(3,686)
Medallion Bank [Member]
Debt Securities, Available-for-sale [Line Items]
Investment income				26,660		52,989
Interest expense				3,186		6,293
Net interest income				23,474		46,696
Noninterest income				37		72
Operating expenses				6,650		12,700
Net investment income before income taxes				16,861		34,068
Income tax (provision)				(1,638)		(4,095)
Net investment income after income taxes				15,223		29,973
Net realized/unrealized losses of Medallion Bank				(13,306)		(23,728)
Unrealized depreciation on Medallion Bank	[1]			(592)		(620)
Net realized/unrealized gains (losses) on controlled subsidiaries other than Medallion Bank				(395)		3,429
Net decrease in net assets resulting from operations				930		9,054
Medallion Bank [Member] | Medallion Financing Trust I [Member]
Debt Securities, Available-for-sale [Line Items]
Net decrease in net assets resulting from operations				$ 1,917		$ 6,245

[1] Unrealized depreciation on Medallion 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 Medallion Bank.