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Allowance for Credit Losses - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Sep. 30, 2024 USD ($) payment	Sep. 30, 2023 USD ($)	Jul. 01, 2006
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable allowance period increase (decrease)			$ (30,000,000)
Loan sale reduction to provision			$ 235,955,000	$ 136,531,000
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%
Maximum amount of forbearance granted			12 months
Number of monthly payments required by a borrower between successive grants of forbearance | payment			12
Interest rate reduction		2.00%
Threshold period for payment default			60 days
Period of loans past due that have accrued interest			90 days
Monthly payment that is smaller than the interest accrued on the loan in that month			$ 25
Extended Maturity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, excluding accrued interest, modified, charged off | payment			300,000
Private Education Loans | Extended Maturity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, excluding accrued interest, modified, accumulated		$ 8,200,000
Financial receivable, excluding accrued interest, modified in period, to total financing receivables, percentage		0.04%
Financing receivable, modified, weighted average term increase from modification		6 years 8 months 12 days
Financial receivable, excluding accrued interest, modified, subsequent default			$ 1,100,000
Financing receivable, excluding accrued interest, modified, subsequent default, unpaid principal balance			$ 1,200,000
Financing receivable, excluding accrued interest, modified, charged off | payment			200,000
Loan delinquent 30-59 days | Private Education Loans | Extended Maturity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, excluding accrued interest, modified, accumulated	$ 700,000		$ 700,000
Loan delinquent 60-89 days | Private Education Loans | Extended Maturity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, excluding accrued interest, modified, accumulated	300,000		300,000
Loan delinquent 90 days or greater past due | Private Education Loans | Extended Maturity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, excluding accrued interest, modified, accumulated	400,000		400,000
Loan delinquent, current | Private Education Loans | Extended Maturity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, excluding accrued interest, modified, accumulated	6,800,000		6,800,000
Loan delinquent, 60 days | Private Education Loans | Extended Maturity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, excluding accrued interest, modified, subsequent default	500,000
Financing receivable, excluding accrued interest, modified, subsequent default, unpaid principal balance	$ 500,000
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan sale reduction to provision			235,955,000	136,531,000
Loans sold			$ 3,690,000,000	2,100,000,000
Threshold period when delinquent loans are written off			120 days
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan sale reduction to provision			$ 0	$ 0
Percentage of FFELP loans insured and guaranteed (at least)			97.00%
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increments in which forbearance is granted			2 months
Interest rate offered to borrowers facing financial difficulty		8.00%
Temporary interest rate reduction, period		4 years
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increments in which forbearance is granted			1 month
Interest rate offered to borrowers facing financial difficulty		2.00%
Temporary interest rate reduction, period		2 years

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