Delinquencies as a percentage of loans in repayment.

No definition available.

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

- Definition

Loan status forbearance recorded investment.

+ References

No definition available.

+ Details

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

- Definition

Loan status in school grace and deferment recorded investment.

+ References

No definition available.

+ Details

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

- Definition

Loans in forbearance as a percentage of loans in repayment and forbearance.

+ References

No definition available.

+ Details

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

- Definition

Total loans in repayment.

+ References

No definition available.

+ Details

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

- Definition

Loans in repayment and percentage of each status.

+ References

No definition available.

+ Details

Name:	slm_LoansInRepaymentAndPercentageOfEachStatusAbstract
Namespace Prefix:	slm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Loans in repayment current in percentage.

+ References

No definition available.

+ Details

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

- Definition

Total loans in repayment, in percentage.

+ References

No definition available.

+ Details

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

- Definition

Percentage of loans in repayment.

+ References

No definition available.

+ Details

Name:	slm_PercentageOfLoansInRepayment
Namespace Prefix:	slm_
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_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of financing receivable balance that is past due.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivablePercentPastDue1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of financing receivable not past due.

+ References

+ Details

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

- Definition

Amount of financing receivable past due.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=118955255&loc=SL82922355-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919253-210447

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953401-111524

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=118939070&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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

- Definition

Amount, before allowance and fees, of loans and leases receivable. Excludes loans and leases covered under loss sharing agreements.

+ References

+ Details

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

+ Details

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

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6378536&loc=d3e10092-111533

+ Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=slm_PersonalLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables30To59DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables60To89DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=slm_PrivateEducationLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Loans in repayment and percentage of each status:
Private Education Loans allowance for losses	$ (414,406)	$ (383,997)	$ (341,121)	$ (328,974)	$ (295,277)	$ (251,475)
Loans held for investment, net	24,716,664		22,270,919
Personal Loans
Loans in repayment and percentage of each status:
Total Private Education Loans, gross	1,131,833		1,190,091
Private Education Loans deferred origination costs	594		297
Private Education Loans allowance for losses	(70,173)		(62,201)
Loans held for investment, net	1,062,254		1,128,187
Personal Loans \| Consumer Portfolio Segment
Loans in repayment and percentage of each status:
Loans current	$ 1,109,512		$ 1,172,776
Loans current	98.00%		98.50%
Total TDR loans in repayment	$ 1,131,833		$ 1,190,091
Total TDR loans in repayment	100.00%		100.00%
Total Private Education Loans, gross	$ 1,131,833		$ 1,190,091
Private Education Loans deferred origination costs	594		297
Total Private Education Loans	1,132,427		1,190,388
Private Education Loans allowance for losses	(70,173)		(62,201)
Loans held for investment, net	$ 1,062,254		$ 1,128,187
Delinquencies as a percentage of Private Education Loans in repayment	2.00%		1.50%
Personal Loans \| Consumer Portfolio Segment \| Loans delinquent 31-60 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 6,341		$ 6,722
Loans delinquent	0.60%		0.60%
Personal Loans \| Consumer Portfolio Segment \| Loans delinquent 61-90 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 8,382		$ 5,416
Loans delinquent	0.70%		0.50%
Personal Loans \| Consumer Portfolio Segment \| Loans delinquent, greater than 90 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 7,598		$ 5,177
Loans delinquent	0.70%		0.40%
Private Education Loans
Loans in repayment and percentage of each status:
Total Private Education Loans, gross	$ 23,120,169		$ 20,504,465
Private Education Loans deferred origination costs	78,103		68,321
Private Education Loans allowance for losses	(342,544)		(277,943)
Loans held for investment, net	22,855,728		20,294,843
Private Education Loans \| Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in-school/grace/deferment	6,443,910		5,260,445
Loans in forbearance	603,280		577,164
Loans in repayment and percentage of each status:
Loans current	$ 15,627,722		$ 14,289,705
Loans current	97.20%		97.40%
Total TDR loans in repayment	$ 16,072,979		$ 14,666,856
Total TDR loans in repayment	100.00%		100.00%
Total Private Education Loans, gross	$ 23,120,169		$ 20,504,465
Private Education Loans deferred origination costs	78,103		68,321
Total Private Education Loans	23,198,272		20,572,786
Private Education Loans allowance for losses	(342,544)		(277,943)
Loans held for investment, net	$ 22,855,728		$ 20,294,843
Percentage of Private Education Loans in repayment	69.50%		71.50%
Delinquencies as a percentage of Private Education Loans in repayment	2.80%		2.60%
Loans in forbearance as a percentage of Private Education Loans in repayment and forbearance	3.60%		3.80%
Private Education Loans \| Consumer Portfolio Segment \| Loans delinquent 31-60 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 263,331		$ 231,216
Loans delinquent	1.60%		1.60%
Private Education Loans \| Consumer Portfolio Segment \| Loans delinquent 61-90 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 120,007		$ 95,105
Loans delinquent	0.80%		0.70%
Private Education Loans \| Consumer Portfolio Segment \| Loans delinquent, greater than 90 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 61,919		$ 50,830
Loans delinquent	0.40%		0.30%