Allowance for Loan Losses - Amortized Cost Basis of Loans Modified to Borrowers with Financial Difficulty (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost			$ 68		$ 186
Private Education Loans [Member] | Interest Rate Reduction [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 592			$ 1,488
Loan Modifications, % of Loan Type	3.30%			8.30%
Loans receivable, weighted average rate of interest	5.50%	13.40%		5.20%		13.10%
Private Education Loans [Member] | More Than an Insignificant Payment Delay [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 305			$ 773
Loan Modifications, % of Loan Type	1.70%			4.30%
Loans receivable, weighted average period added to remaining life of loan	6 months			6 months
Private Education Loans [Member] | Combination Rate Reduction and Term Extension [Member]
Accounts Notes And Loans Receivable [Line Items]
Loan Modifications, Amortized Cost	$ 42			$ 119
Loan Modifications, % of Loan Type	0.20%			0.70%
Loans receivable, weighted average rate of interest	5.40%	13.00%		5.20%		12.60%
Loans receivable, weighted average period added to remaining life of loan	7 years			8 years