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Defined Contribution Plans (Details) - Sallie Mae 401(k) Savings Plan - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items] |  |  |
Type 1 of defined benefit contribution (up to) | 3.00% |  |
Type 2 of defined contribution plan | 2.00% |  |
Percent of core employer contribution | 1.00% |  |
Employer contribution amount | $ 5.0 | $ 4.4 | $ 3.8
Maximum |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Employer matching contribution percentage | 100.00% |  |
Minimum |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Employer matching contribution percentage | 50.00% |  |

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- Definition Defined Contribution Plan, Requisite Service Period, Core Employer Contribution + References No definition available. + Details
Name: | slm_DefinedContributionPlanRequisiteServicePeriodCoreEmployerContribution
Namespace Prefix: | slm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Type 1 of Defined Contribution Plan + References No definition available. + Details
Name: | slm_Type1ofDefinedContributionPlan
Namespace Prefix: | slm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Type 2 of Defined Contribution Plan + References No definition available. + Details
Name: | slm_Type2ofDefinedContributionPlan
Namespace Prefix: | slm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |