Debt Instrument, Unused Borrowing Capacity Fee, Percentage

No definition available.

Name:	slm_DebtInstrumentUnusedBorrowingCapacityFeePercentage
Namespace Prefix:	slm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Estimated weighted average life of student loans.

+ References

No definition available.

+ Details

Name:	slm_EstimatedWeightedAverageLifeOfStudentLoans
Namespace Prefix:	slm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Lendable value of collateral

+ References

No definition available.

+ Details

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

- Definition

Loans Securitized or Asset-Backed Financing Arrangement, Ownership Interest, Percentage

+ References

No definition available.

+ Details

Name:	slm_LoansSecuritizedorAssetBackedFinancingArrangementOwnershipInterestPercentage
Namespace Prefix:	slm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ownership Interest Percentage

+ References

No definition available.

+ Details

Name:	slm_OwnershipInterestPercentage
Namespace Prefix:	slm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Uncommitted Federal Funds

+ References

No definition available.

+ Details

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

- Definition

Principal amount of transferred financial assets in which the transferor has continuing involvement with the assets underlying the transaction. Continuing involvement includes, but is not limited to, servicing, recourse, and restrictions on transferor's interests in transferred financial assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107314-111719

+ Details

Name:	us-gaap_ContinuingInvolvementWithTransferredFinancialAssetsPrincipalAmountOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

+ Details

Name:	us-gaap_LoansPledgedAsCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This is the principal amount outstanding for loans which are in the process of being securitized as of the date of the statement of financial position (across all types of loans).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107314-111719

+ Details

Name:	us-gaap_PrincipalAmountOutstandingOnLoansHeldForSaleOrSecuritization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from the issuance of long-term debt that is not secured by collateral. Excludes proceeds from tax exempt unsecured debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfUnsecuredDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_ShortTermBorrowings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=slm_ABCPBorrowingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=slm_PrivateEducationLoans2018TermAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=slm_ClassAandBNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=slm_PrivateEducationLoans2018TermBMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=slm_PrivateEducationLoans2018TermCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_UnsecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=slm_SeniorUnsecuredNotesDueApril52022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Borrowings - Additional Information (Details) - USD ($)					9 Months Ended		12 Months Ended
Borrowings - Additional Information (Details) - USD ($)	Sep. 19, 2018	Jun. 20, 2018	Mar. 21, 2018	Feb. 21, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Apr. 05, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Short-term borrowings					$ 0		$ 0
Unsecured offering issued					1,900,500,000		$ 1,448,000,000		$ 1,782,000,000
Proceeds from loan securitization sold to third parties					$ 0	$ 197,000,000
Estimated weighted average life of student loans (in years)					5 years 4 months 24 days		5 years 6 months
Uncommitted federal funds					$ 125,000,000
Lendable value of collateral					3,100,000,000		$ 2,600,000,000
ABCP borrowings \| Commercial Paper
Line of Credit Facility [Line Items]
Private asset backed commercial paper education loan funding facility				$ 750,000,000
Ownership interest percentage in residual interest in ABCP facility				100.00%
Short-term borrowings					0		0
Private Education Loans 2018 Term A
Line of Credit Facility [Line Items]
Total loan amount securitized at inception			$ 670,000,000
Amount of loan securitization sold to third parties			$ 670,000,000
Ownership interest percentage			100.00%
Proceeds from loan securitization sold to third parties			$ 668,000,000
Loans pledged as collateral					661,000,000
Class A and B Notes
Line of Credit Facility [Line Items]
Estimated weighted average life of student loans (in years)	4 years 3 months 25 days	4 years 4 months 24 days	4 years 5 months 4 days
Private Education Loans 2018 Term B
Line of Credit Facility [Line Items]
Total loan amount securitized at inception		$ 687,000,000
Amount of loan securitization sold to third parties		$ 687,000,000
Ownership interest percentage		100.00%
Proceeds from loan securitization sold to third parties		$ 683,000,000
Loans pledged as collateral					692,000,000
Private Education Loans 2018 Term C
Line of Credit Facility [Line Items]
Total loan amount securitized at inception	$ 544,000,000
Amount of loan securitization sold to third parties	$ 544,000,000
Ownership interest percentage	100.00%
Proceeds from loan securitization sold to third parties	$ 541,000,000
Loans pledged as collateral					555,000,000
LIBOR \| ABCP borrowings \| Commercial Paper
Line of Credit Facility [Line Items]
Basis spread on variable rate				0.85%
LIBOR \| Class A and B Notes
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.77%	0.76%	0.78%
Minimum \| ABCP borrowings \| Commercial Paper
Line of Credit Facility [Line Items]
Financing cost percentage of unused borrowing capacity				0.35%
Maximum \| ABCP borrowings \| Commercial Paper
Line of Credit Facility [Line Items]
Financing cost percentage of unused borrowing capacity				0.45%
Unsecured borrowings
Line of Credit Facility [Line Items]
Short-term borrowings					0		$ 0
Unsecured borrowings \| Senior Unsecured Notes Due April 5, 2022
Line of Credit Facility [Line Items]
Unsecured offering issued					$ 197,000,000			$ 200,000,000
Interest rate stated percentage								5.125%