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Loans Held for Investment - Student Loan Portfolio Average Balances (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Receivables [Abstract]    
Average Balance, Private Education Loans $ 9,454,579slm_AveragebalanceofPrivateEducationLoans $ 7,419,714slm_AveragebalanceofPrivateEducationLoans
Weighted Average Interest Rate, Private Education Loans 8.07%slm_WeightedAverageInterestRateofPrivateEducationLoans 8.14%slm_WeightedAverageInterestRateofPrivateEducationLoans
Average Balance FFELP Loans 1,234,682slm_AverageBalanceFfelpLoans 1,404,595slm_AverageBalanceFfelpLoans
Weighted Average Interest Rate FFELP loans 3.19%slm_WeightedAverageInterestRateFfelploans 3.19%slm_WeightedAverageInterestRateFfelploans
Average balance, total portfolio $ 10,689,261slm_LoansandLeaseReceivableNetAverageBalance $ 8,824,309slm_LoansandLeaseReceivableNetAverageBalance