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Commitments, Contingencies and Guarantees - Additional Information (Detail) (USD $)
Dec. 31, 2014
Sep. 29, 2014
Commitments and Contingencies Disclosure [Abstract]    
FDIC civil monetary penalties $ 3,300,000slm_FDICcivilmonetarypenalties $ 3,300,000slm_FDICcivilmonetarypenalties
Contingency refund $ 30,000,000slm_Contingencyrefund $ 30,000,000slm_Contingencyrefund