Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,179,021)	$ (1,361,723)	$ (1,673,324)	$ (441,912)
Loans held for investment, net	20,368,287	19,183,143
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	20,741,840	19,729,337
Deferred origination costs and unamortized premium/(discount)	64,468	63,475
Allowance for credit losses	(1,173,375)	(1,355,844)
Loans held for investment, net	19,632,933	18,436,968
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	9,504,760	8,950,216
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	11,237,080	10,779,121
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	727,664	737,593
Deferred origination costs and unamortized premium/(discount)	1,954	1,993
Allowance for credit losses	(4,318)	(4,378)
Loans held for investment, net	725,300	735,208
Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	11,309	12,238
Deferred origination costs and unamortized premium/(discount)	73	230
Allowance for credit losses	(1,328)	(1,501)
Loans held for investment, net	$ 10,054	$ 10,967