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Allowance for Loan Losses - Modified Loan Accounts for TDR (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Receivables [Abstract] |  |  |  |
Modified Loans | $ 134,489 | $ 92,782 | $ 246,695 | $ 153,848
Charge-offs | 12,215 | 5,464 | 22,738 | 10,432
Payment- Default | $ 23,679 | $ 21,388 | $ 49,113 | $ 47,089

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- Definition Charge Offs In TDR Portfolio + References No definition available. + Details
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Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Modified Loans + References No definition available. + Details
Name: | slm_ModifiedLoans
Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Troubled Debt Restructurings Payment Default + References No definition available. + Details
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Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_ReceivablesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration