Loans Held for Investment -Narrative (Details)	3 Months Ended	12 Months Ended
	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($) payment	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Finance Receivable Transferred To Held For Sale [Line Items]
Private loans indexed to LIBOR		33.00%	45.00%
Tier 1 of government guarantee (at least)		97.00%			97.00%
Tier 2 of government guarantee						98.00%
Tier 3 of government guarantee							100.00%
Gain (loss) on sale of financing receivable		$ 164,000,000	$ 328,000,000	$ 548,000,000
Proceeds from sale of loans receivable		3,150,000,000	3,340,000,000	4,240,000,000
Net proceeds from sales of loans held for investment, principal		2,930,000,000	3,130,000,000	3,980,000,000
Net proceeds from sales of loans held for investment, capitalized interest		$ 226,000,000	$ 217,000,000	$ 264,000,000
Estimated weighted average life of student loans		5 years	5 years
Number of payments due before granting forbearance | payment		1
Increment in which disaster forbearance is granted		3 months
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 excluded from disallowance of forbearance following grace period		6 months
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		43.50%	43.10%
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	$ (4,000,000)
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent		$ 151,000,000	$ 135,000,000
Loans pledged to borrower in custody		1,400,000,000	2,700,000,000
FFELP Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent		$ 45,000,000	$ 68,000,000
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%
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%