Loans and Allowance for Loan Losses - Summary of Non Accrual Loan (Detail) - USD ($) $ in Thousands	6 Months Ended	9 Months Ended			12 Months Ended
	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	[1]	Dec. 31, 2017	[2]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total nonaccrual loans	$ 47,904	$ 45,765	$ 132,316		$ 98,494
Interest foregone quarter to date	770	563	1,845		823
Amount of foregone interest applied to principal in the quarter	400	350	574		52
Interest foregone life to date	8,281	8,530	16,286		12,485
Amount of foregone interest applied to principal life to date	$ 3,748	$ 3,412	$ 9,750		$ 3,495
Percentage of nonaccrual loans to gross loan portfolio	4.00%	4.00%	36.00%		31.00%
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non accrual loans		$ 39,626
Interest collected on nonaccrual loans recorded		1,278
Interest paid		1,102
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non accrual loans		32,668
Interest collected on nonaccrual loans recorded		1,487
Interest paid		$ 1,221

[1] Does not include Medallion Bank nonaccrual loans of $39,626, $1,278 of interest income foregone and $1,102 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.