Loans and Allowance for Loan Losses - Summary of Non Accrual Loan (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended			12 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	[1]	Dec. 31, 2017	[2]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total nonaccrual loans	$ 47,904	$ 47,904	$ 122,042		$ 98,494
Interest foregone quarter to date		770	2,248		823
Amount of foregone interest applied to principal in the quarter		400	679		52
Interest foregone life to date		8,281	14,934		12,485
Amount of foregone interest applied to principal life to date		$ 3,748	$ 9,711		$ 3,495
Percentage of nonaccrual loans to gross loan portfolio	4.00%	4.00%	34.00%		31.00%
Interest collected on nonaccrual loans recorded	$ 309
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non accrual loans	43,246	$ 43,246
Interest collected on nonaccrual loans recorded		1,379
Interest paid		1,065
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non accrual loans	$ 32,668	32,668
Interest collected on nonaccrual loans recorded		1,487
Interest paid		$ 1,221

[1] Does not include Medallion Bank: nonaccrual loans of $43,246, $1,379 of interest income foregone and $1,065 of foregone interest paid and applied to principal.

[2] Does not include Medallion Bank: nonaccrual loans of $32,668, $1,487 of interest income foregone and $1,221 of foregone interest paid and applied to principal.