Loans Held for Investment - Additional Information (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2021 USD ($) payment	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2021 USD ($) payment numberOfExtensions rateReduction	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Finance Receivable Transferred To Held For Sale [Line Items]
Private loans indexed to LIBOR	52.10%				55.00%				52.10%	55.00%
Tier 1 of government guarantee												97.00%
Tier 2 of government guarantee													98.00%
Tier 3 of government guarantee														100.00%
Gains on sales of loans, net	$ 145,535,000	$ (10,000)	$ 3,679,000	$ 399,111,000	$ (247,000)	$ (4,000)	$ (369,000)	$ 238,935,000	$ 548,315,000	$ 238,315,000	$ 0
Proceeds from sale of loans receivable									4,240,000,000	3,100,000,000
Net proceeds from sales of loans held for investment, principal									3,980,000,000	2,900,000,000
Net proceeds from sales of loans held for investment, capitalized interest									$ 264,000,000	$ 199,000,000
Net proceeds from sales of personal loans, principal						697,000,000
Net proceeds from personal loans, accrued interest						7,000,000
Estimated weighted average life of student loans									4 years 8 months 12 days	5 years 4 months 24 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 | numberOfExtensions									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
Loans currently in repayment status that were subject to interest rate reductions	7.20%				7.80%				7.20%	7.80%
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	38.40%				38.80%				38.40%	38.80%
Percentage of aggregate outstanding loans held for investment									5.00%
Personal Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Loan sale reduction to provision						$ 43,000,000
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent	$ 80,000,000				$ 81,000,000				$ 80,000,000	$ 81,000,000
Loans pledged to borrower in custody	2,900,000,000				3,300,000,000				2,900,000,000	3,300,000,000
FFELP Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent	$ 34,000,000				$ 36,000,000				$ 34,000,000	$ 36,000,000
Maximum
Finance Receivable Transferred To Held For Sale [Line Items]
Increments in which forbearance is granted									2 months
Increase to life of loan defaults									16.60%
Minimum
Finance Receivable Transferred To Held For Sale [Line Items]
Increments in which forbearance is granted									1 month
Increase to life of loan defaults									10.10%