Income Taxes - Summary of Components of Deferred and Other Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Goodwill and other intangibles/unrealized gain on investments in Medallion Bank		$ (45,736)	$ (35,297)
Provision for loan losses/unrealized losses on loans and nonaccrual interest		27,002	10,071
Net operating loss carryforwards	[1]	17,062	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		1,815	782
Unrealized gains on investments and other assets		(3,877)	(542)
Total deferred tax liability		(3,734)	(29,383)
Valuation allowance		(167)	(39)
Deferred tax liability, net		(3,901)	(29,422)
Taxes receivable		1,890	16,886
Net deferred and other tax liabilities		$ (2,011)	$ (12,536)

[1] As of September 30, 2018, various subsidiaries of the Company had $11,148 of net operating loss carryforwards that expire at various dates between December 31, 2026 and December 31, 2035, which had a net asset value of $2,057 as of the balance sheet date.