Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,160,244)	$ (1,179,021)	$ (1,361,723)	$ (1,929,323)	$ (1,673,324)	$ (441,912)
Loans held for investment, net	20,115,144		19,183,143
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	20,477,757		19,729,337
Deferred origination costs and unamortized premium/(discount)	65,872		63,475
Allowance for credit losses	(1,154,540)		(1,355,844)
Loans held for investment, net	19,389,089		18,436,968
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	9,442,904		8,950,216
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	11,034,853		10,779,121
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	716,958		737,593
Deferred origination costs and unamortized premium/(discount)	1,913		1,993
Allowance for credit losses	(4,262)		(4,378)
Loans held for investment, net	714,609		735,208
Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	12,784		12,238
Deferred origination costs and unamortized premium/(discount)	104		230
Allowance for credit losses	(1,442)		(1,501)
Loans held for investment, net	$ 11,446		$ 10,967