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

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- Definition Average Balance Ffelp Loans + References No definition available. + Details
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Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Average balance of Private Education Loans + References No definition available. + Details
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Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Loans and Lease Receivable, Net, Average Balance + References No definition available. + Details
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Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition No authoritative reference available. + References No definition available. + Details
Name: | slm_WeightedAverageInterestRateFfelploans
Namespace Prefix: | slm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition No authoritative reference available. + References No definition available. + Details
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Namespace Prefix: | slm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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