Allowance for Credit Losses -Narrative (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2023 USD ($) payment	Dec. 31, 2022 USD ($)	Jul. 01, 2006
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Expense (reversal) financial receivable and off-balance sheet	$ 15,599,000	$ 198,023,000	$ 17,729,000	$ 114,112,000	$ 297,260,000	$ 207,598,000	$ 30,545,000	$ 98,050,000	$ 345,000,000	$ 633,000,000
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
Threshold period for payment default									60 days
Period of loans past due that have accrued interest									90 days	90 days
Monthly payment that is smaller than the interest accrued on the loan in that month									$ 25
Private Education Loans
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Loans sold									$ 3,150,000,000	$ 3,340,000,000
Threshold period when delinquent loans are written off									120 days
FFELP Loans
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Percentage of FFELP loans insured and guaranteed (at least)									97.00%
Maximum
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Interest rate offered to borrowers facing financial difficulty	8.00%								8.00%
Increments in which forbearance is granted									2 months
Minimum
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Interest rate offered to borrowers facing financial difficulty	2.00%								2.00%
Increments in which forbearance is granted									1 month