Allowance for Credit Losses - Narrative (Details)	3 Months Ended	9 Months Ended
	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) termChange payment	Sep. 30, 2022 USD ($)	Jul. 01, 2006
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable allowance period decrease		$ 6,000,000
Negative provisions for loan sale	$ 50,226,000	$ 136,531,000	$ 171,325,000
Interest rate offered to borrowers facing financial difficulty, period		2 years
Number of term changes | termChange		2
Criteria for loans to be considered as nonperforming (greater than)		90 days
Tier 1 of government guarantee (at least)		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
Monthly payment that is smaller than the interest accrued on the loan in that month		$ 25
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Negative provisions for loan sale	50,226,000	136,531,000	171,325,000
Loans sold		$ 2,100,000,000	3,290,000,000
Threshold period when delinquent loans are written off		120 days
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Negative provisions for loan sale	$ 0	$ 0	$ 0
Percentage of FFELP loans insured and guaranteed (at least)		97.00%
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest rate offered to borrowers facing financial difficulty		4.00%
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