Loans and Allowance for Loan Losses - Summary of Activity in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended	9 Months Ended
		Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Provision for loan losses		$ 18,205	$ 48,781	$ 48,781	[1]
Allowance for loan losses - ending balance		29,484	29,484	29,484
Recreation [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - ending balance		2,880	2,880	2,880
Home Improvement [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - ending balance		861	861	861
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - ending balance		100	100	100
Medallion [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - ending balance		25,643	25,643	25,643
Bank Holding Company Accounting [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - beginning balance	[2]	21,425
Total charge- offs		(11,941)		(23,428)
Total recoveries		1,795		4,131
Net charge offs		(10,146)		(19,297)
Provision for loan losses		18,205		48,781
Allowance for loan losses - ending balance	[3]	29,484	$ 29,484	29,484
Bank Holding Company Accounting [Member] | Recreation [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total charge- offs		(4,825)		(9,471)
Total recoveries		1,318		3,217
Bank Holding Company Accounting [Member] | Home Improvement [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total charge- offs		(659)		(1,220)
Total recoveries		367		606
Bank Holding Company Accounting [Member] | Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total recoveries				4
Bank Holding Company Accounting [Member] | Medallion [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total charge- offs		(6,457)		(12,737)
Total recoveries		$ 110		$ 304

[1] Balance includes the six months ended September 30, 2018 under Bank Holding Company Accounting and three months ended March 31, 2018 under Investment Company Accounting.

[2] Beginning balance for the six months September 30, 2018 ended reflects the transition to Bank Holding Company Accounting by netting previously established unrealized depreciation against the gross loan balances resulting in a starting point of zero for this table.

[3] Includes $15,587 of a general reserve for current and performing medallion loans under 90 days past due, as an additional buffer against future losses, representing 53% of the total allowance, and 7% of the loans in question.