Allowance for Loan Losses - Amortized Cost Basis of Loans Modified to Borrowers with Financial Difficulty (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan Modifications, Amortized Cost		$ 250	$ 149
Interest Rate Reduction [Member] | Private Education Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 1,668
Loan Modifications, % of Loan Type	9.50%
Loans receivable, weighted average rate of interest	5.30%	13.20%
More Than an Insignificant Payment Delay [Member] | Private Education Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 923
Loan Modifications, % of Loan Type	5.30%
Loans receivable, weighted average period added to remaining life of loan	7 months
Combination Rate Reduction and Term Extension [Member] | Private Education Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 138
Loan Modifications, % of Loan Type	0.80%
Loans receivable, weighted average rate of interest	5.20%	12.70%
Loans receivable, weighted average period added to remaining life of loan	8 years