Significant Accounting Polices (Details) $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($)
Accounting Policies [Abstract]
Reclassification resulting from the adoption of ASU No. 2018-02	$ 0.6

Amount of increase (decrease) in accumulated other comprehensive income (AOCI) for reclassification to retained earnings of tax effect from remeasurement of deferred tax pursuant to Tax Cuts and Jobs Act of 2017.

No definition available.

Name:	slm_TaxCutsAndJobsActOf2017ReclassificationFromAociToRetainedEarningsTaxEffect
Namespace Prefix:	slm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration