Allowance for Loan Losses - Education Loans - Amortized Cost Basis of Loans Modified to Borrowers with Financial Difficulty (Details) - Private Education Loans [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Interest Rate Concession [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 562	$ 568	$ 1,154	$ 1,171
Loan Modifications, % of Loan Type	3.60%	3.60%	7.50%	7.40%
Loans receivable, weighted average rate of interest	11.60%	12.30%	11.60%	12.40%
Loans receivable, weighted average rate of interest after reduction	5.20%	5.30%	5.20%	5.40%
More Than an Insignificant Payment Delay [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 240	$ 295	$ 473	$ 548
Loan Modifications, % of Loan Type	1.60%	1.90%	3.10%	3.50%
Loans receivable, weighted average period added to remaining life of loan	5 months	5 months	7 months	7 months
Combination Rate Reduction and Term Extension [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 31	$ 35	$ 67	$ 75
Loan Modifications, % of Loan Type	0.20%	0.20%	0.40%	0.50%
Loans receivable, weighted average rate of interest	11.20%	12.10%	11.20%	12.00%
Loans receivable, weighted average rate of interest after reduction	4.70%	5.40%	4.90%	5.40%
Loans receivable, weighted average period added to remaining life of loan	6 years	7 years	7 years	7 years