Loans Held for Investment -Narrative (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($) payment	Dec. 31, 2023 USD ($)
Finance Receivable Transferred To Held For Sale [Line Items]
Percent of private loans indexed to LIBOR or SOFR		23.00%	33.00%
Estimated weighted average life of student loans		5 years 7 months 6 days	5 years
Maximum amount of forbearance granted		12 months
Period of positive payment performance required by a borrower between successive grants of forbearance		12 months
Number of payments required by a borrower between successive grants of forbearance | payment		12
Period that delinquent borrowers can participate in certain short-term interest-only payment alternatives, option one		12 months
Period that delinquent borrowers can participate in certain short-term interest-only payment alternatives, option two		5 years
Interest rate reduction		2.00%
Period of loans past due that have accrued interest		90 days	90 days
Loans in nonaccrual status		$ 0	$ 0
Percentage of loans concentrated in major states		43.80%	43.50%
Percentage of aggregate outstanding loans held for investment		5.00%
Credit Cards
Finance Receivable Transferred To Held For Sale [Line Items]
Gain (loss) on sale of financing receivable	$ (8,000,000)		$ (4,000,000)
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent		$ 142,000,000	151,000,000
Loans pledged to borrower in custody		2,300,000,000	1,400,000,000
FFELP Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Charge-off		$ 8,000,000
Ninety or more days delinquent			$ 45,000,000
Maximum
Finance Receivable Transferred To Held For Sale [Line Items]
Increments in which forbearance is granted		2 months
Interest rate offered to borrowers facing financial difficulty		8.00%
Interest rate offered to borrowers facing financial difficulty, duration		4 years
Minimum
Finance Receivable Transferred To Held For Sale [Line Items]
Increments in which forbearance is granted		1 month
Interest rate offered to borrowers facing financial difficulty		2.00%
Interest rate offered to borrowers facing financial difficulty, duration		2 years