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Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2020 | Mar. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Average Balance | $ 25,252,841 | $ 23,747,242
Private Education Loans |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Average Balance | $ 23,502,844 | $ 21,732,826
Weighted Average Interest Rate | 8.86% | 9.50%
FFELP Loans |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Average Balance | $ 776,326 | $ 837,950
Weighted Average Interest Rate | 4.29% | 4.94%
Personal Loans |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Average Balance | $ 973,671 | $ 1,176,466
Weighted Average Interest Rate | 12.11% | 11.81%

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- Definition Financing Receivable, Before Allowance For Credit Loss, Average Balance + References No definition available. + Details
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Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Weighted Average Interest Rate on Loans + References No definition available. + Details
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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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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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: |