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Allowance for Credit Losses and Unfunded Loan Commitments - Narrative (Details)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2025 USD ($)	Sep. 30, 2025 USD ($) payment	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)	Jul. 01, 2006
Accounts, Notes, Loans and Financing Receivable [Line Items]
Monthly payment that is smaller than the interest accrued on the loan in that month		$ 25
Financing receivable allowance period increase (decrease)		$ 51,000,000
Loan sale reduction to provision			$ 235,955,000
Criteria for loans to be considered as nonperforming (greater than)		90 days
Tier 1 of government guarantee (at least)	97.00%	97.00%		97.00%		97.00%
Maximum amount of forbearance granted		12 months
Number of monthly payments required by a borrower between successive grants of forbearance | payment		12
Threshold period for payment default			60 days
Financing receivable, modified in period, subsequently charged off, amount		$ 31,100,000		$ 34,400,000	$ 40,400,000
Period of loans past due that have accrued interest		90 days
Contractual obligation	$ 2,700,000,000	$ 2,700,000,000		2,700,000,000
Private Education Loans | Extended Maturity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, excluding accrued interest, modified, accumulated	5,800,000	$ 5,800,000		$ 5,800,000
Financial receivable, excluding accrued interest, modified in period, to total financing receivables, percentage		0.02%
Financing receivable, modified, weighted average term increase from modification		7 years
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan sale reduction to provision	118,794,000	$ 235,253,000	$ 235,955,000
Loans sold		$ 3,940,000,000	$ 3,690,000,000
Threshold period when delinquent loans are written off		120 days
Financing receivable, modified in period, subsequently charged off, amount	$ 12,100,000	$ 31,100,000
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increments in which forbearance is granted		2 months
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increments in which forbearance is granted		1 month

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