Allowance for Credit Losses and Unfunded Loan Commitments - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2024 USD ($) payment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Expense (reversal) financial receivable and off-balance sheet		$ 409,000,000	$ 345,000,000
Financing receivable, excluding accrued interest, allowance for credit loss, loan sold		$ 235,955,000	205,383,000	$ 174,231,000
Criteria for loans to be considered as nonperforming (greater than)		90 days
Tier 1 of government guarantee (at least)		97.00%
Maximum amount of forbearance granted		12 months
Number of monthly payments required by a borrower between successive grants of forbearance | payment		12
Financing receivable, ,modified in period, amount		$ 1,001,410,000	373,683,000
Amortization cost basis		$ 40,400,000	$ 8,400,000
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
Loan delinquent, current
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Financing receivable, ,modified in period, amount		826,007,000	$ 334,967,000
Loan delinquent 30-59 days
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Financing receivable, ,modified in period, amount		77,446,000	17,205,000
Loan delinquent 60-89 days
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Financing receivable, ,modified in period, amount		43,484,000	7,689,000
Loan delinquent 90 days or greater past due
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Financing receivable, ,modified in period, amount		54,473,000	13,822,000
Private Education Loans | Extended Maturity
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Financing Receivable, excluding accrued interest, modified, accumulated	$ 7,300,000		7,300,000
Financing receivable, excluding accrued interest, modified in period, to total financing receivables, percentage	0.03%
Financing receivable, modified, weighted average term increase for modification	6 years 8 months 12 days
Financing receivable, modified, accumulated	$ 2,500,000	2,000,000.0	2,500,000
Financing receivable, ,modified in period, amount	$ 600,000
Private Education Loans | Extended Maturity | Loan delinquent, current
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Financing Receivable, excluding accrued interest, modified, accumulated		6,000,000.0
Private Education Loans | Extended Maturity | Loan delinquent 30-59 days
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Financing Receivable, excluding accrued interest, modified, accumulated		600,000
Private Education Loans | Extended Maturity | Loan delinquent 60-89 days
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Financing Receivable, excluding accrued interest, modified, accumulated		400,000
Private Education Loans | Extended Maturity | Loan delinquent 90 days or greater past due
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Financing Receivable, excluding accrued interest, modified, accumulated		300,000
Private Education Loans
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Financing receivable, excluding accrued interest, allowance for credit loss, loan sold		235,955,000	205,383,000	174,231,000
Loans sold		3,690,000,000	3,150,000,000	3,340,000,000
Financing receivable, ,modified in period, amount		1,035,055,000	380,526,000
Amortization cost basis		$ 40,400,000
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]
Financing receivable, excluding accrued interest, allowance for credit loss, loan sold		$ 0	$ 0	$ 0
Maximum
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Increments in which forbearance is granted		2 months
Minimum
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Increments in which forbearance is granted		1 month