Contractual Maturity

No definition available.

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

- Definition

Debt Instrument, Contractual Maturity, Term, Amortization Period

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument, Contractual Maturity, Term, Revolving Period

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument, Unused Borrowing Capacity Fee, Percentage

+ References

No definition available.

+ Details

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_DebtInstrumentLineItems
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_DebtInstrumentAxis=slm_PrivateEducationLoans2018TermAMember
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_DebtInstrumentAxis=slm_PrivateEducationLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=slm_PrivateEducationLoans2017TermBMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=slm_PrivateEducationLoans2017TermAMember
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_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
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:	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 (Details) - USD ($)							12 Months Ended
Borrowings - Additional (Details) - USD ($)	Sep. 19, 2018	Jun. 20, 2018	Mar. 21, 2018	Feb. 21, 2018	Nov. 08, 2017	Feb. 08, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Apr. 05, 2017
Debt Instrument [Line Items]
Amount drawn and outstanding under ABCP facility							$ 0	$ 0
Total Issued							12,453,903,000	1,448,000,000		$ 1,900,500,000
Unsecured debt issued							$ 0	$ 197,000,000	$ 0
Estimated weighted average life of student loans							5 years 5 months 5 days	5 years 6 months
Discretionary uncommitted federal funds line of credit							$ 125,000,000
Lendable value of collateral							3,100,000,000	$ 2,600,000,000
ABCP borrowings
Debt Instrument [Line Items]
Amount drawn and outstanding under ABCP facility							$ 0	0
Contractual maturity related to ABCP facility							2 years
Revolving period of contractual maturity							1 year
Amortization period of contractual maturity							1 year
Private Education Loans 2018 Term A [Member]
Debt Instrument [Line Items]
Private education loan term accounted for as a secured financing			$ 670,000,000
Ownership interest percentage on asset-backed financing			100.00%
Loans sold to third parties			$ 670,000,000
Unsecured debt issued			$ 668,000,000
Loans pledged as collateral							$ 647,000,000
Private Education Loans 2018 Term B [Member]
Debt Instrument [Line Items]
Private education loan term accounted for as a secured financing		$ 687,000,000
Ownership interest percentage on asset-backed financing		100.00%
Loans sold to third parties		$ 687,000,000
Unsecured debt issued		$ 683,000,000
Loans pledged as collateral							678,000,000
Private Education Loans 2018 Term C [Member]
Debt Instrument [Line Items]
Private education loan term accounted for as a secured financing	$ 544,000,000
Ownership interest percentage on asset-backed financing	100.00%
Loans sold to third parties	$ 544,000,000
Unsecured debt issued	$ 541,000,000
Loans pledged as collateral							544,000,000
Private Education Loans
Debt Instrument [Line Items]
Private education loan term accounted for as a secured financing					$ 676,000,000	$ 772,000,000	1,900,000,000
Ownership interest percentage on asset-backed financing					100.00%	100.00%
Unsecured debt issued					$ 674,000,000	$ 768,000,000
Private Education Loans, 2017 Term B
Debt Instrument [Line Items]
Loans pledged as collateral							617,000,000
ABS Transactions
Debt Instrument [Line Items]
Private education loan term accounted for as a secured financing									$ 2,400,000,000
Private Education Loans, 2017 Term A
Debt Instrument [Line Items]
Loans pledged as collateral							631,000,000
Class A and B Notes
Debt Instrument [Line Items]
Estimated weighted average life of student loans	4 years 3 months 26 days	4 years 4 months 25 days	4 years 5 months 4 days		4 years 26 days	4 years 3 months 8 days
London Interbank Offered Rate (LIBOR) \| Class A and B Notes
Debt Instrument [Line Items]
Basis spread on variable rate	0.77%	0.76%	0.78%		0.80%	0.93%
Commercial Paper \| ABCP borrowings
Debt Instrument [Line Items]
Private education loan funding				$ 750,000,000.0
Ownership Interest Percentage				100.00%
Amount drawn and outstanding under ABCP facility							0	0
Commercial Paper \| London Interbank Offered Rate (LIBOR) \| ABCP borrowings
Debt Instrument [Line Items]
Basis spread on variable rate				0.85%
Minimum \| Commercial Paper \| ABCP borrowings
Debt Instrument [Line Items]
Unused borrowing capacity fee (as a percent)				0.35%
Maximum \| Commercial Paper \| ABCP borrowings
Debt Instrument [Line Items]
Unused borrowing capacity fee (as a percent)				0.45%
Unsecured Debt
Debt Instrument [Line Items]
Amount drawn and outstanding under ABCP facility							0	$ 0
Total Issued							200,000,000
Unsecured Debt \| Senior Unsecured Notes Due April 5, 2022
Debt Instrument [Line Items]
Total Issued							$ 197,000,000				$ 200,000,000
Interest rate											5.125%