Allowance for Credit Losses - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jul. 01, 2006
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reduction to provision for credit losses	$ 427,000,000
Moody's Analytics Base and S4 downturn scenarios, applicable weight in determining allowance for credit losses	50.00%
Interest rate offered to borrowers facing financial difficulty	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	9.20%		8.80%
Percentage of loans granted forbearance (as a percentage)	47.00%	47.00%
Criteria for loans to be considered as nonperforming (greater than)	90 days
Tier 1 of government guarantee (at least)	97.00%			97.00%
TDR payment default period (more than)	60 days
Percentage of FFELP loans insured and guaranteed (at least)	97.00%
Period of loans past due that have accrued interest (greater than)	90 days
Monthly payment that is smaller than the interest accrued on the loan in that month	$ 25
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest rate offered to borrowers facing financial difficulty	4.00%