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Borrowed Funds Borrowed Funds (Details) (USD $) | Mar. 31, 2015 | Dec. 31, 2014
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Debt Disclosure [Abstract] |  |
Uncommitted Fed Funds | $ 100,000,000 slm_UncommittedFederalFunds |
Lendable value of collateral | $ 1,361,879,000 slm_Lendablevalueofcollateral | $ 1,398,286,000 slm_Lendablevalueofcollateral

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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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- Details
Name: | us-gaap_DebtDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration