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Borrowings - Other Borrowing Sources Narrative (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
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Debt Disclosure [Abstract] |  |
Uncommitted federal funds |  | $ 125
Lendable value of collateral | $ 2,200 | $ 1,600

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- 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

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- 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

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