Allowance for Loan Losses - 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		12 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022
Interest Rate Concession [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 601		$ 548		$ 1,143	$ 1,161
Loan Modifications, % of Loan Type	3.60%		3.00%		6.80%	6.30%
Loans receivable, weighted average rate of interest	5.50%	13.30%	5.30%	13.00%	5.50%	5.10%	13.30%	12.90%
More Than an Insignificant Payment Delay [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 314		$ 274		$ 597	$ 539
Loan Modifications, % of Loan Type	1.90%		1.50%		3.60%	2.90%
Loans receivable, weighted average period added to remaining life of loan	5 months		5 months		6 months	6 months
Combination Rate Reduction and Term Extension [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 39		$ 45		$ 78	$ 91
Loan Modifications, % of Loan Type	0.20%		0.20%		0.50%	0.50%
Loans receivable, weighted average rate of interest	5.30%	12.70%	5.30%	12.40%	5.30%	5.20%	12.80%	12.40%
Loans receivable, weighted average period added to remaining life of loan	7 years		8 years		7 years	8 years