Significant Accounting Policies - Allowance for FFELP Loans 2019 (Details)	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Percentage of loans granted forbearance (as a percentage)	47.00%	50.00%
Percentage reimbursement on all qualifying default claims period one (as a percentage)	98.00%	98.00%
Variable rate	100.00%	100.00%

Loans and Leases Receivable, Impaired, Loans Granted Forbearance Qualified as Trouble Debt Restructuring

No definition available.

Name:	slm_LoansandLeasesReceivableImpairedLoansGrantedForbearanceQualifiedasTroubleDebtRestructuring
Namespace Prefix:	slm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant