Loans and Allowance for Credit Losses - Summary of Inclusive Capitalized Loans (Detail) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2024		Dec. 31, 2023		Mar. 31, 2024		Jun. 30, 2023		Mar. 31, 2023		Dec. 31, 2022
Student Loan Portfolio By Program [Line Items]
Total gross loans	$ 2,385,590		$ 2,215,886				$ 2,156,998
Allowance for credit losses	(89,788)	[1]	(84,235)	[2]	$ (83,827)	[2]	(74,971)	[1]	$ (70,280)	[2]	$ (63,845)	[2]
Net loans receivable	2,295,802		2,131,651
Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	2,385,590		2,215,886		2,228,426		2,156,998		1,984,180		1,916,953
Allowance for credit losses	89,788		84,235
Net loans receivable	$ 2,295,802		$ 2,131,651
Percentage of total gross loans	100.00%		100.00%
Recreation [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	$ 1,497,428		$ 1,336,226
Allowance for credit losses	(65,140)		(57,532)
Recreation [Member] | Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	$ 1,497,428		$ 1,336,226		1,365,165		1,331,114		1,213,380		1,183,512
Percentage of total gross loans	63.00%		60.00%
Home Improvement [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	$ 773,184		$ 760,617
Allowance for credit losses	(18,388)		(21,019)
Home Improvement [Member] | Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	$ 773,184		$ 760,617		752,262		728,468		669,642		626,399
Percentage of total gross loans	32.00%		34.00%
Commercial [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	$ 110,197		$ 114,827
Allowance for credit losses	(4,861)		(4,148)
Commercial [Member] | Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	$ 110,197		$ 114,827		106,570		92,637		95,329		92,899
Percentage of total gross loans	5.00%		5.00%
Taxi Medallion [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	$ 3,482		$ 3,663
Allowance for credit losses	(1,399)		(1,536)
Taxi Medallion [Member] | Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	3,482		3,663		3,560		3,448		4,059
Strategic Partnership [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	1,299		553
Strategic Partnership [Member] | Bank Holding Company Accounting [Member]
Student Loan Portfolio By Program [Line Items]
Total gross loans	$ 1,299		$ 553		$ 869		$ 1,331		$ 1,770		$ 572

[1] As of June 30, 2024 and June 30, 2023, there were no allowance for credit losses and net charge-offs related to the strategic partnership loans.

[2] 2023 beginning balance represents allowance prior to the adoption of ASU 2016-13.