Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,227,362)	$ (1,165,335)	$ (1,179,021)	$ (1,361,723)
Loans held for investment, net	21,291,675	20,341,283
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	21,735,369	20,716,863
Deferred origination costs and unamortized premium/(discount)	71,907	67,488
Allowance for credit losses	(1,221,053)	(1,158,977)
Loans held for investment, net	20,586,223	19,625,374
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	10,685,053	9,920,547
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	11,050,316	10,796,316
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	682,273	695,216
Deferred origination costs and unamortized premium/(discount)	1,770	1,815
Allowance for credit losses	(3,999)	(4,077)
Loans held for investment, net	680,044	692,954
Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	27,547	25,014
Deferred origination costs and unamortized premium/(discount)	171	222
Allowance for credit losses	(2,310)	(2,281)
Loans held for investment, net	$ 25,408	$ 22,955