Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,081,066)	$ (1,227,362)	$ (1,165,335)	$ (1,160,244)	$ (1,179,021)	$ (1,361,723)
Loans held for investment, net	19,201,328		20,341,283
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	19,519,185		20,716,863
Deferred origination costs and unamortized premium/(discount)	66,809		67,488
Allowance for credit losses	(1,074,744)		(1,158,977)
Loans held for investment, net	18,511,250		19,625,374
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	9,748,824		9,920,547
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	9,770,361		10,796,316
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	665,666		695,216
Deferred origination costs and unamortized premium/(discount)	1,715		1,815
Allowance for credit losses	(3,929)		(4,077)
Loans held for investment, net	663,452		692,954
Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	28,900		25,014
Deferred origination costs and unamortized premium/(discount)	119		222
Allowance for credit losses	(2,393)		(2,281)
Loans held for investment, net	$ 26,626		$ 22,955