Loans Held for Investment -Narrative (Details)	3 Months Ended									12 Months Ended
	Dec. 31, 2022 USD ($) payment	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2022 USD ($) numberOfExtension payment rateReduction	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Finance Receivable Transferred To Held For Sale [Line Items]
Private loans indexed to LIBOR	45.00%				52.00%					45.00%	52.00%
Tier 1 of government guarantee (at least)	97.00%									97.00%			97.00%
Tier 2 of government guarantee														98.00%
Tier 3 of government guarantee															100.00%
Net proceeds from sales of personal loans, principal									$ 697,000,000
Net proceeds from personal loans, accrued interest									7,000,000
Gains on sales of loans, net	$ 2,894,000	$ 74,978,000	$ 239,997,000	$ 9,881,000	$ 145,535,000	$ (10,000)	$ 3,679,000	$ 399,111,000		$ 327,750,000	$ 548,315,000	$ 238,315,000
Proceeds from sale of loans receivable										3,340,000,000	4,240,000,000	3,100,000,000
Net proceeds from sales of loans held for investment, principal										3,130,000,000	3,980,000,000	2,900,000,000
Net proceeds from sales of loans held for investment, capitalized interest										$ 217,000,000	$ 264,000,000	$ 199,000,000
Estimated weighted average life of student loans										5 years	4 years 8 months 12 days
Number of payments due before granting forbearance | payment	1									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										4.00%
Interest rate offered to borrowers facing financial difficulty, period										2 years
Number of available extensions of final maturity date of loan under loan modification program, borrowers facing financial difficulty | numberOfExtension										1
Number of rate reductions available under loan modification program, borrowers facing financial difficulty | rateReduction										2
Interest rate offered to borrowers facing financial difficulty	4.00%									4.00%
Number of consecutive monthly payments required to qualify for rate reduction, borrowers facing financial difficulty | payment										3
Period of positive payment performance required by a borrower between successive grants of forbearance, borrowers facing financial difficulty										12 months
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	43.10%				38.40%					43.10%	38.40%
Percentage of aggregate outstanding loans held for investment										5.00%
Florida
Finance Receivable Transferred To Held For Sale [Line Items]
Percentage of loans held in state, less than					500.00%						500.00%
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent	$ 135,000,000				$ 80,000,000					$ 135,000,000	$ 80,000,000
Loans pledged to borrower in custody	2,700,000,000				2,900,000,000					2,700,000,000	2,900,000,000
FFELP Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent	$ 68,000,000				$ 34,000,000					$ 68,000,000	$ 34,000,000
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	4.00%									4.00%
Minimum
Finance Receivable Transferred To Held For Sale [Line Items]
Increments in which forbearance is granted										1 month
Personal Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Loan sale reduction to provision									$ 43,000,000