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Borrowings - Other Borrowing Sources (Details) - USD ($) | Dec. 31, 2020 | Dec. 31, 2019
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Debt Disclosure [Abstract] |  |
Discretionary uncommitted federal funds line of credit | $ 125,000,000 |
Lendable value of collateral | $ 3,800,000,000 | $ 3,200,000,000

X
- Definition Lendable value of collateral + References No definition available. + Details
Name: | slm_Lendablevalueofcollateral
Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Uncommitted Federal Funds + References No definition available. + Details
Name: | slm_UncommittedFederalFunds
Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_DebtDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration