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Allowance for Loan Losses - Loan Type (Detail) - FFELP Loans [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024
Financing Receivable Recorded Investment Past Due [Line Items]
Total loans, gross	$ 29,800	$ 33,134
Change	(3,334)
Stafford Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total loans, gross	9,703	10,589
Change	(886)
Consolidation Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total loans, gross	17,077	19,273
Change	(2,196)
Rehab Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total loans, gross	3,020	$ 3,272
Change	$ (252)

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