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Allowance for Loan Losses - Modified Loan Accounts for TDR (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Receivables [Abstract] |  |  |
Modified Loans | $ 394,639 | $ 498,812 | $ 398,324
Charge-offs | 52,823 | 48,469 | 24,628
Payment-Default | $ 90,231 | $ 92,532 | $ 64,811

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- Definition Charge Offs In TDR Portfolio + References No definition available. + Details
Name: | slm_ChargeOffsInTDRPortfolio
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
Name: | slm_TroubledDebtRestructuringsPaymentDefault
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