Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,215,407)	$ (1,160,244)	$ (1,361,723)	$ (1,734,559)	$ (1,929,323)	$ (441,912)
Loans held for investment, net	21,281,527		19,183,143
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	21,702,837		19,729,337
Deferred origination costs and unamortized premium/(discount)	68,584		63,475
Allowance for credit losses	(1,209,460)		(1,355,844)
Loans held for investment, net	20,561,961		18,436,968
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	10,306,885		8,950,216
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	11,395,952		10,779,121
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	705,691		737,593
Deferred origination costs and unamortized premium/(discount)	1,870		1,993
Allowance for credit losses	(4,206)		(4,378)
Loans held for investment, net	703,355		735,208
Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	17,766		12,238
Deferred origination costs and unamortized premium/(discount)	186		230
Allowance for credit losses	(1,741)		(1,501)
Loans held for investment, net	$ 16,211		$ 10,967