Allowance for Loan Losses - Loan Type (Detail) - FFELP Loans [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable Recorded Investment Past Due [Line Items]
Total Loans, gross	$ 38,140	$ 43,747	$ 52,903
Change	(5,607)
Stafford Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Loans, gross	12,171	14,019
Change	(1,848)
Consolidation Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Loans, gross	22,272	25,522
Change	(3,250)
Rehab Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Loans, gross	3,697	$ 4,206
Change	$ (509)