Regulatory Capital - U.S. Basel III Regulatory Requirements (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Actual Amount
Common Equity Tier 1 Capital (to Risk-Weighted Assets)	$ 3,063,308	$ 2,957,067
Tier 1 Capital (to Risk-Weighted Assets)	3,063,308	2,957,067
Total Capital (to Risk-Weighted Assets)	3,418,923	3,294,663
Tier 1 Capital (to Average Assets)	$ 3,063,308	$ 2,957,067
Actual Ratio
Common Equity Tier 1 Capital (to Risk-Weighted Assets)	11.30%	11.30%
Tier 1 Capital (to Risk-Weighted Assets)	0.113	0.113
Total Capital (to Risk-Weighted Assets)	0.126	0.126
Tier 1 Capital (to Average Assets)	0.104	0.097
U.S. Basel III Minimum Requirements Plus Buffer, Amount
Common Equity Tier 1 Capital (to Risk-Weighted Assets)	$ 1,902,074	$ 1,827,318
Tier 1 Capital (to Risk-Weighted Assets)	2,309,661	2,218,886
Total Capital (to Risk-Weighted Assets)	2,853,110	2,740,976
Tier 1 Capital (to Average Assets)	$ 1,177,879	$ 1,213,505
U.S. Basel III Minimum Requirements Plus Buffer, Ratio
Common Equity Tier 1 Capital (to Risk-Weighted Assets)	7.00%	7.00%
Tier 1 Capital (to Risk-Weighted Assets)	0.085	0.085
Total Capital (to Risk-Weighted Assets)	0.105	0.105
Tier 1 Capital (to Average Assets)	0.040	0.040

Common Equity Tier One Risk Based Capital

No definition available.

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

- Definition

Tier One Common Equity Capital Required To Be Well Capitalized To Risk Weighted Assets

+ References

No definition available.

+ Details

Name:	slm_TierOneCommonEquityCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	slm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier One Common Equity Capital to Risk Weighted Assets

+ References

No definition available.

+ Details

Name:	slm_TierOneCommonEquityCapitaltoRiskWeightedAssets
Namespace Prefix:	slm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier One Risk Based Capital For Common Equity Required To Be Capitalized

+ References

No definition available.

+ Details

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

- Definition

Amount of total capital as defined by regulatory framework.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of minimum total risk-based capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

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

- Definition

Ratio of minimum total risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of total risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of Tier 1 leverage capital as defined by regulatory framework.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of minimum Tier 1 leverage capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

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

- Definition

Ratio of minimum Tier 1 leverage capital to average assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of Tier 1 capital to average assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of Tier 1 risk-based capital as defined by regulatory framework.

+ References

+ Details

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

- Definition

Amount of minimum Tier 1 risk-based capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

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

- Definition

Ratio of minimum Tier 1 risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of Tier 1 risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant