Allowance for Credit Losses - Narrative (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) payment	Jun. 30, 2023 USD ($)	Jul. 01, 2006
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable allowance period increase (decrease)				$ (103,000,000)
Loan sale reduction to provision	$ 102,751,000		$ 136,531,000	$ 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				0
Private Education Loans | Extended Maturity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, excluding accrued interest, modified, accumulated		$ 9,300,000
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	$ 900,000			$ 1,000,000.0
Financing receivable, excluding accrued interest, modified, subsequent default, unpaid principal balance	800,000			$ 900,000
Threshold period for payment default				60 days
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	600,000			$ 600,000
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	500,000			500,000
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	300,000			300,000
Loan delinquent, current | Private Education Loans | Extended Maturity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, excluding accrued interest, modified, accumulated	7,900,000			7,900,000
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan sale reduction to provision	102,751,000		136,531,000	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	$ 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