Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)			3 Months Ended				6 Months Ended	9 Months Ended			12 Months Ended
		Jan. 01, 2016	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Non-marketable securities			$ 10,752,000				$ 10,752,000	$ 10,752,000
Investment in non-marketable securities											$ 9,521,000
Net premium on investment securities			186,000							$ 250,000
Investment securities Amortized to interest income			26,000			$ 21,000	47,000			61,000
Net loan origination costs								14,041,000			90,000
Net accretion to income			(1,147,000)			(17,000)		(2,192,000)	[1]	(55,000)
Total loans more than 90 days past due			50,695,000				50,695,000	50,695,000			88,886,000
Loans pledged as collateral			123,173,000				123,173,000	123,173,000			183,529,000
Principal portion of loans serviced, fair value			26,558,000				26,558,000	$ 26,558,000			338,867,000
Net unrealized appreciation on investments, net of tax					$ 172,177,000	100,732,000				100,732,000	139,700,000	$ 110,374,000	$ 122,595,000	$ 127,367,000
Intangible assets useful life								20 years
Amortization of intangible assets	[1]							$ 722,000
Depreciation and amortization					$ 23,000	23,000				71,000
Amortization expense			558,000			229,000		1,322,000		697,000
Deferred costs			4,859,000			3,295,000	$ 4,859,000	$ 4,859,000		$ 3,295,000	$ 3,070,000
Potential dilutive common shares excluded from EPS computation								115,000		359,000
Stock based compensation award								39,000		23,333
Stock based compensation award, Amount			$ 151,000			$ 222,000		$ 446,000		$ 551,000
Stock based compensation award per diluted common share			$ 0.02			$ 0.02	$ 0.02	$ 0.02		$ 0.02
Unrecognized compensation cost related to unvested stock options and restricted stock			$ 408,000				$ 408,000	$ 408,000
Unrecognized compensation cost related to unvested stock options and restricted stock, recognition period								3 years
Interest rate cap expense			$ 0			$ 0		$ 0		$ 19,000
Tier 1 leverage capital ratio			15.08%				15.08%	15.08%
Capital conversation buffer		0.625%
Period increase of capital conversation buffer		0.625%
Common Equity Tier 1 risk-based capital ratio			13.70%				13.70%	13.70%			13.80%
Tier 1 risk-based capital ratio			16.30%				16.30%	16.30%			16.50%
Total risk-based capital ratio			17.70%				17.70%	17.70%			17.80%
Restricted Shares [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock based compensation award			2,846	212	97,952			101,010		258,232	327,251
Medallion Financing Trust I [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Appreciation in Investment in Medallion Bank											$ 152,267,000
Principal portion of loans serviced, fair value, Medallion bank											311,988,000
91+ [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans more than 90 days past due			$ 14,061,000				$ 14,061,000	$ 14,061,000			60,450,000
91+ [Member] | Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans more than 90 days past due			$ 14,061,000				$ 14,061,000	$ 14,061,000			$ 60,450,000
Total loans more than 90 days past due ,percentage			1.29%				1.29%	1.29%			18.90%
Medallion Bank [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Appreciation in Investment in Medallion Bank					$ 39,826,000						$ 7,849,000			$ 128,918,000	$ 15,500,000
Net loan origination costs								$ 11,187,000			11,187,000
Net accretion to income			$ 901,000			901,000		3,065,000		$ 2,526,000
Reserves against future losses								15,587,000
Amortization expense			567,000			567,000		1,680,000		1,680,000
Deferred costs			$ 5,011,000			5,437,000	$ 5,011,000	$ 5,011,000		5,437,000	5,011,000
Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of fixed assets								3 years
Tier 1 leverage capital to total assets ratio			15.00%				15.00%	15.00%
Common Equity Tier 1 risk-based capital ratio			7.00%				7.00%	7.00%
Tier 1 risk-based capital ratio			8.50%				8.50%	8.50%
Total risk-based capital ratio			10.50%				10.50%	10.50%
Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of fixed assets								10 years
Medallion Bank and Other Controlled Subsidiaries [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in non-marketable securities											302,147,000
Investment Company Accounting [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net accretion to income										(55,000)
Net unrealized appreciation on investments, net of tax						100,732,000				100,732,000	$ 139,700,000
Bank Holding Company Accounting [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Goodwill and intangible assets			$ 210,761,000				$ 210,761,000	$ 210,761,000
Amortization of intangible assets			361,000					722,000
Interest Rate Cap [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract amount to purchase interest rate caps			20,000,000				20,000,000	20,000,000
Interest rate cap carried to Balance Sheet			0				$ 0	0
Leasehold Improvements [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Depreciation and amortization			131,000			23,000		289,000		71,000
Leasehold Improvements [Member] | Medallion Bank [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Depreciation and amortization			$ 64,000			$ 64,000		$ 166,000		$ 166,000

[1] Balance includes the six months ended September 30, 2018 under Bank Holding Company Accounting and three months ended March 31, 2018 under Investment Company Accounting.