Loans Held for Investment - Additional Information (Details) $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) payment	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) payment repaymentAlternative numberOfExtensions rateReduction	Sep. 30, 2020 USD ($)	Dec. 31, 2020	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent of private loans indexed to LIBOR	53.00%				53.00%		55.00%
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%
Gain from sale | $	$ (10)			$ (4)	$ 402,780	$ 238,562
Proceeds from sale of loans receivable | $					3,190,000
Net proceeds from sales of loans held for investment, principal | $		$ 25,000	$ 2,970,000		2,990,000
Net proceeds from sales of loans held for investment, capitalized interest | $		$ 2,000	$ 193,000		$ 195,000
Estimated weighted average life of student loans					4 years 8 months 12 days		5 years 4 months 24 days
Maximum amount of forbearance granted					12 months
Number of payments due before granting forbearance | payment	1				1
Increment in which disaster forbearance is granted					3 months
Period of positive payment performance required by a borrower between successive grants of forbearance					6 months
Number of payments required by a borrower between successive grants of forbearance | payment					6
Period excluded from disallowance of forbearance following grace period					6 months
Number of times delinquent borrowers can participate in certain short-term interest-only payment alternatives in a 12 month period | repaymentAlternative					2
Period that delinquent borrowers can participate in certain short-term interest-only payment alternatives, option one					12 months
Number of times delinquent borrowers can participate in certain short-term interest-only payment alternatives in a five-year period | repaymentAlternative					1
Period that delinquent borrowers can participate in certain short-term interest-only payment alternatives, option two					5 years
Period of positive payment performance required by a borrower between successive grants of forbearance, updated credit administration practices					12 months
Number of payments required by a borrower between successive grants of forbearance, updated credit administration practices | payment					12
Interest rate offered to borrowers facing financial difficulty	4.00%				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
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					6 months
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	9.20%				9.20%		7.80%
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increments in which forbearance is granted					1 month
Loans expected to have accelerated default rates and increases to loan life					10.10%
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increments in which forbearance is granted					2 months
Interest rate offered to borrowers facing financial difficulty	4.00%				4.00%
Loans expected to have accelerated default rates and increases to loan life					16.60%