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Allowance for Credit Losses - Accrued Interest Receivable (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
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Receivables [Abstract] |  |
Total Interest Receivable | $ 1,248,475 | $ 1,168,895
90 Days and Greater Past Due | 3,695 | 4,354
Allowance for Uncollectible Interest | $ 4,405 | $ 4,467

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- Definition Accrued interest receivable on private education loans. + References No definition available. + Details
Name: | slm_AccruedInterestReceivableOnPrivateEducationLoans
Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Allowance For Uncollectible Accrued Interest + References No definition available. + Details
Name: | slm_AllowanceForUncollectibleAccruedInterest
Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Greater than ninety days past due accrued interest receivable. + References No definition available. + Details
Name: | slm_GreaterThanNinetyDaysPastDueAccruedInterestReceivable
Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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