Loans and Allowance for Loan Losses - Summary of Inclusive Capitalized Loans (Detail) - USD ($)		6 Months Ended		12 Months Ended
		Jun. 30, 2020		Dec. 31, 2019		Mar. 31, 2020	Jun. 30, 2019		Mar. 31, 2019	Dec. 31, 2018
Student Loan Portfolio By Program [Line Items]
Total gross loans		$ 1,234,687,000	[1]	$ 1,136,694,000	[2]
Allowance for loan losses		(66,977,000)	[3],[4]	(46,093,000)		$ (54,057,000)	$ (40,670,000)	[3],[4]	$ (36,862,000)	$ (36,395,000)
Net loans receivable		1,193,617,000		1,114,762,000			1,047,805,000
Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans		1,260,594,000		1,160,855,000		1,183,779,000	1,088,475,000		1,024,200,000	1,017,882,000
Allowance for loan losses		(66,977,000)		(46,093,000)
Net loans receivable		$ 1,193,617,000		$ 1,114,762,000
Percentage of total gross loans		100.00%		100.00%
Recreation [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans		$ 761,403,000	[1]	$ 689,810,000	[2]
Allowance for loan losses		(27,021,000)		(18,075,000)
Recreation [Member] | Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans		$ 786,785,000		$ 713,332,000		735,175,000	668,540,000		609,999,000	587,038,000
Percentage of total gross loans		62.00%		62.00%
Home Improvement [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans		$ 285,455,000	[1]	$ 250,830,000	[2]
Allowance for loan losses		(4,072,000)		(2,608,000)
Home Improvement [Member] | Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans		$ 282,072,000		$ 247,324,000		255,899,000	209,549,000		193,275,000	183,155,000
Percentage of total gross loans		22.00%		21.00%
Commercial [Member] | Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans		$ 71,476,000		$ 69,767,000		68,257,000	64,442,000		55,211,000	64,083,000
Percentage of total gross loans		6.00%		6.00%
Medallion [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans		$ 116,345,000	[1],[5]	$ 126,287,000	[2]
Allowance for loan losses		(35,884,000)		(25,410,000)
Medallion [Member] | Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans		$ 120,253,000		$ 130,432,000		$ 124,448,000	145,944,000		$ 165,715,000	$ 183,606,000
Percentage of total gross loans		10.00%		11.00%
Strategic Partnership [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	[1]	$ 8,000
Allowance for loan losses		0					$ 0
Strategic Partnership [Member] | Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans		$ 8,000

[1] Excludes loan premiums of $5,251 resulting from purchase price accounting and $20,656 of capitalized loan origination costs.

[2] Excludes loan premiums of $5,758 resulting from purchase price accounting and $18,403 of capitalized loan origination costs.

[3] As of June 30, 2020, there was no allowance for loan loss and net charge-offs related to the strategic partnership loans.

[4] Includes $2,025 of a general reserve for the Company, for current and performing medallion loans under 90 days past due, as an additional buffer against future losses, representing 3% of the total allowance, and 1.94% of the medallion loans under 90 days past due as of June 30, 2020. This figure excludes $17,351 of a general reserve on loans at the Bank, which was netted against loan balances at consolidation on April 2, 2018. Subsequent to April 2, 2018, the Bank recorded a general reserve benefit of $11,555.

[5] Included in the current medallion loan aging bucket were $80,403 of loans that had been granted deferral status under the CARES Act, some or all of which may have become delinquent had they not been granted the deferral status.