Income Taxes - Summary of Components of Deferred and Other Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Bank Holding Company Accounting [Member]
Income Tax Rate Reconciliation [Line Items]
Goodwill and other intangibles/unrealized gain on investments in Medallion Bank		$ (46,089)
Provision for loan losses/unrealized losses on loans and nonaccrual interest		31,152
Net operating loss carryforwards	[1]	2,133
Accrued expenses, compensation, and other		1,218
Unrealized gains on investments and other assets		(3,958)
Total deferred tax liability		(15,544)
Valuation allowance		(108)
Deferred tax liability, net		(15,652)
Taxes receivable		19,112
Net deferred and other tax assets		$ 3,460
Investment Company Accounting [Member]
Income Tax Rate Reconciliation [Line Items]
Goodwill and other intangibles/unrealized gain on investments in Medallion Bank			$ (35,297)
Provision for loan losses/unrealized losses on loans and nonaccrual interest			10,071
Net operating loss carryforwards	[1]		615
Unrealized gains on investments in other controlled subsidiaries			(3,617)
Unrealized gains on investments other than securities			(1,395)
Accrued expenses, compensation, and other			782
Unrealized gains on investments and other assets			(542)
Total deferred tax liability			(29,383)
Valuation allowance			(39)
Deferred tax liability, net			(29,422)
Taxes receivable			16,886
Net deferred and other tax assets			$ (12,536)

[1] As of June 30, 2018, the Company had $11,148 of net operating loss carryforwards that expire at various dates between December 31, 2026 and December 31, 2036.