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		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Interest Rate Reductions [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 388	$ 599	$ 1,082	$ 1,554
Loan Modifications, % of Loan Type	3.00%	3.80%	8.30%	9.80%
Loans receivable, weighted average rate of interest	11.90%	12.30%	11.90%	12.30%
Loans receivable, weighted average rate of interest after reduction	5.10%	5.40%	5.20%	5.40%
More Than an Insignificant Payment Delay [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 198	$ 298	$ 489	$ 734
Loan Modifications, % of Loan Type	1.50%	1.90%	3.80%	4.60%
Loans receivable, weighted average period added to remaining life of loan	5 months	5 months	6 months	7 months
Combination Rate Reduction and Term Extension [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 18	$ 33	$ 62	$ 87
Loan Modifications, % of Loan Type	0.10%	0.20%	0.50%	0.50%
Loans receivable, weighted average rate of interest	10.70%	12.00%	11.20%	12.10%
Loans receivable, weighted average rate of interest after reduction	4.70%	5.20%	4.70%	5.40%
Loans receivable, weighted average period added to remaining life of loan	6 years	6 years	6 years	7 years