Allowance for Credit Losses - Additional Information (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Jul. 01, 2006
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Less: provisions for credit losses									$ (232,708,000)	$ 190,898,000
Decrease to provisions									126,000,000
Proceeds from sale of loans receivable									$ 4,240,000,000	$ 3,100,000,000
Interest rate offered to borrowers facing financial difficulty	4.00%								4.00%
Interest rate offered to borrowers facing financial difficulty, period									2 years
Percentage of loans in full and interest repayment status that are subject to interest rate reductions	7.20%				7.80%				7.20%	7.80%
Percentage of loans granted forbearance (as a percentage)	47.00%				47.00%				47.00%	47.00%
Criteria for loans to be considered as nonperforming									90 days
Tier 1 of government guarantee (at least)												97.00%
TDR payment default period									60 days
Period of loans past due that have accrued interest									90 days	90 days
Monthly payment that is smaller than the interest accrued on the loan in that month									$ 25
Less: provisions for credit losses	$ (15,309,000)	$ 138,442,000	$ 69,677,000	$ (225,767,000)	$ (316,372,000)	$ (3,640,000)	$ 351,887,000	$ 61,258,000	(32,957,000)	$ 93,133,000	$ 354,249,000
Private Education Loans
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Less: provisions for credit losses									(233,852,000)	148,673,000
Less: provisions for credit losses									(34,101,000)	93,824,000
FFELP Loans
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Less: provisions for credit losses									$ 20,000	$ 412,000
Tier 1 of government guarantee (at least)	97.00%								97.00%
Maximum
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Interest rate offered to borrowers facing financial difficulty	4.00%								4.00%