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Allowance for Loan Losses - Accrued Interest Receivable (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
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Receivables [Abstract] |  |
Total Interest Receivable | $ 1,258,751 | $ 1,366,158
Greater Than 90 Days Past Due | 3,127 | 2,390
Allowance for Uncollectible Interest | $ 3,479 | $ 5,309

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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