Allowance for Credit Losses - Narrative (Details)	3 Months Ended
	Mar. 31, 2023 USD ($) payment termChange	Jul. 01, 2006
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable allowance period increase	$ 16,000,000
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]
Threshold period when delinquent loans are written off	120 days
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
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