Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,194,238)	$ (1,081,066)	$ (1,165,335)	$ (1,215,407)	$ (1,160,244)	$ (1,361,723)
Loans held for investment, net	19,622,302		20,341,283
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	20,104,463		20,716,863
Deferred origination costs and unamortized premium/(discount)	66,816		67,488
Allowance for credit losses	(1,190,427)		(1,158,977)
Loans held for investment, net	18,980,852		19,625,374
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	10,693,123		9,920,547
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	9,411,340		10,796,316
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	643,614		695,216
Deferred origination costs and unamortized premium/(discount)	1,647		1,815
Allowance for credit losses	(3,811)		(4,077)
Loans held for investment, net	641,450		692,954
Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	0		25,014
Deferred origination costs and unamortized premium/(discount)	0		222
Allowance for credit losses	0		(2,281)
Loans held for investment, net	$ 0		$ 22,955