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Allowance for Loan Losses - Modified Loan Accounts for TDR (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Receivables [Abstract] |  |  |  |
Modified Loans | $ 168,645 | $ 116,419 | $ 415,341 | $ 270,266
Charge-offs | 12,227 | 5,925 | 34,965 | 16,357
Payment- Default | $ 28,275 | $ 23,326 | $ 77,248 | $ 70,401

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- Definition Charge Offs In TDR Portfolio + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Modified Loans + 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 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration