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Borrowed Funds Borrowed Funds (Details) - USD ($) | Jun. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Debt Disclosure [Abstract] |  |
Uncommitted federal funds | $ 100,000,000 |
Lendable value of collateral | $ 1,500,000,000 | $ 1,400,000,000

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