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Significant Accounting Polices - Narrative (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2018 USD ($)
Accounting Policies [Abstract] |
Reclassification from accumulated other comprehensive income to retained earnings resulting from tax cuts and jobs act | $ 0.6

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- Definition Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Reclassification from Accumulated Other Comprehensive Income to Retained Earnings, Provisional Income Tax Expense (Benefit) + References No definition available. + Details
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Namespace Prefix: | slm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration