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Allowance for Loan Losses - Accrued Interest Receivable (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Receivables [Abstract]
Total Interest Receivable	$ 1,276,825	$ 1,168,823
Greater Than 90 Days Past Due	2,374	1,920
Allowance for Uncollectible Interest	$ 4,687	$ 6,322

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- Definition

Accrued interest receivable on private education loans.

+ References

No definition available.

+ Details

Name:	slm_AccruedInterestReceivableOnPrivateEducationLoans
Namespace Prefix:	slm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Allowance For Uncollectible Accrued Interest

+ References

No definition available.

+ Details

Name:	slm_AllowanceForUncollectibleAccruedInterest
Namespace Prefix:	slm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

Greater than ninety days past due accrued interest receivable.

+ References

No definition available.

+ Details

Name:	slm_GreaterThanNinetyDaysPastDueAccruedInterestReceivable
Namespace Prefix:	slm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

Name:	us-gaap_ReceivablesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration