Loans and Allowance for Loan Losses - Summary of Activity in Allowance for Loan Losses (Parenthetical) (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
Percentage of Allowance	100.00%
Medallion Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Reserves against future losses	$ 15,587
Percentage of Allowance	53.00%
Percentage of total gross loans	7.00%
Financing Receivables, 90 Days Past Due [Member] | Medallion Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Reserves against future losses	$ 15,587