Allowance for Loan Losses - Loan Status and Aging of Past Due TDR Loans (Details) - Student Loan - Consumer Portfolio Segment - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
TDR loans in repayment and percentage of each status:
Total Private Education Loans in repayment, 2020	$ 666,681	$ 2,355,395
Total	14,190,329	16,315,651
Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	80,591	87,749
TDR loans in repayment and percentage of each status:
TDR loans in forbearance - current	233,871	99,054
Total TDR loans in forbearance	$ 257,661	$ 99,054
Loans in forbearance, current	90.70%	100.00%
Total TDR loans in forbearance	100.00%	100.00%
Loans current	$ 1,070,807	$ 1,230,954
Total Private Education Loans in repayment, 2020	$ 1,181,988	$ 1,395,163
Loans current	90.60%	88.20%
Total TDR loans in repayment	100.00%	100.00%
Total	$ 1,520,240	$ 1,581,966
Loan delinquency 31-60 days
TDR loans in repayment and percentage of each status:
Total	166,584	288,051
Loan delinquency 31-60 days \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
TDR loans in forbearance - delinquent	$ 11,797	$ 0
Loans in forbearance, delinquent	4.60%	0.00%
Loans delinquent	$ 49,098	$ 85,555
Loans delinquent	4.20%	6.10%
Loan delinquency 61-90
TDR loans in repayment and percentage of each status:
Total	$ 99,653	$ 121,302
Loan delinquency 61-90 \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
TDR loans in forbearance - delinquent	$ 7,906	$ 0
Loans in forbearance, delinquent	3.10%	0.00%
Loans delinquent	$ 35,544	$ 49,626
Loans delinquent	3.00%	3.60%
Loan delinquency greater than 90 days
TDR loans in repayment and percentage of each status:
Total	$ 56,157	$ 62,666
Loan delinquency greater than 90 days \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
TDR loans in forbearance - delinquent	$ 4,087	$ 0
Loans in forbearance, delinquent	1.60%	0.00%
Loans delinquent	$ 26,539	$ 29,028
Loans delinquent	2.20%	2.10%

Financing Receivable, Loans In Forbearance, Not Past Due

No definition available.

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

- Definition

Financing Receivable, Loans In Forbearance, Originated In Current Fiscal Year

+ References

No definition available.

+ Details

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

- Definition

Financing Receivable, Loans In Forbearance, Past Due

+ References

No definition available.

+ Details

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

- Definition

Financing Receivable, Loans In Forbearance, Percent Past Due

+ References

No definition available.

+ Details

Name:	slm_FinancingReceivableLoansInForbearancePercentPastDue
Namespace Prefix:	slm_
Data Type:	num:percentItemType
Balance Type:	na
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, Current, Percentage

+ References

No definition available.

+ Details

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

- Definition

Loans In Forebearance, Percentage

+ References

No definition available.

+ Details

Name:	slm_LoansInForebearancePercentage
Namespace Prefix:	slm_
Data Type:	num:percentItemType
Balance Type:	na
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

Total loans in repayment.

+ References

No definition available.

+ Details

Name:	slm_LoansInRepaymentOriginatedInCurrentFiscalYear
Namespace Prefix:	slm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953401-111524

Reference 2: 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=121647567&loc=SL82922355-210448

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919253-210447

+ 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=121647567&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=121599337&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=121611835&loc=SL6953401-111524

+ Details

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

- Definition

Amount, before allowance for credit loss, of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919230-210447

Reference 3: 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=121647567&loc=SL82922355-210448

+ Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_StudentLoanMember
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_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=slm_TroubledDebtRestructuredLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=slm_FinancingReceivables31to60DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=slm_FinancingReceivables61to90DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=slm_FinancingReceivablesGreaterThan90DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: