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Commitments, Contingencies and Guarantees - Additional Information (Detail) $ in Millions | 3 Months Ended
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Mar. 31, 2018 USD ($)
Commitments and Contingencies Disclosure [Abstract] |
Contractual obligation | $ 500.0
Other liabilities reserve | $ 0.3
Loss contingency, loss emergence period | 1 year

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- Definition Loss Contingency, Loss Emergence Period + References No definition available. + Details
Name: | slm_LossContingencyLossEmergencePeriod
Namespace Prefix: | slm_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Other Liabilities Reserve + References No definition available. + Details
Name: | slm_OtherLiabilitiesReserve
Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- Definition Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments. + References No definition available. + Details
Name: | us-gaap_ContractualObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant