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Borrowed Funds Borrowed Funds (Details) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Debt Disclosure [Abstract]    
Uncommitted Fed Funds $ 100,000,000slm_UncommittedFederalFunds  
Lendable value of collateral $ 1,398,286,000slm_Lendablevalueofcollateral $ 900,217,000slm_Lendablevalueofcollateral