Significant Accounting Polices - Narrative (Details) $ in Millions	3 Months Ended
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

Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Reclassification from Accumulated Other Comprehensive Income to Retained Earnings, Provisional Income Tax Expense (Benefit)

No definition available.

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Namespace Prefix:	slm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration