Regulatory Capital - Well Capitalized Regulatory Requirements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Actual
Tier I Capital (to Average Assets)	$ 1,482,856us-gaap_TierOneLeverageCapital	$ 1,413,988us-gaap_TierOneLeverageCapital
Tier I Capital (to Risk Weighted Assets)	1,482,856us-gaap_TierOneRiskBasedCapital	1,413,988us-gaap_TierOneRiskBasedCapital
Total Capital (to Risk Weighted Assets)	1,572,661us-gaap_Capital	1,497,830us-gaap_Capital
Common Equity Tier I Capital (to Risk Weighted Assets)	1,482,856slm_Commonequitytier1capital
Actual Ratio
Tier I Capital (to Average Assets)	11.50%	11.50%
Tier I Capital (to Risk Weighted Assets)	13.60%	15.00%
Total Capital (to Risk Weighted Assets)	14.40%us-gaap_CapitalToRiskWeightedAssets	15.90%us-gaap_CapitalToRiskWeightedAssets
Common Equity Tier I Capital (to Risk Weighted Assets)	13.60%
Well Capitalized Regulatory Requirements, Amount
Tier I Capital (to Average Assets)	642,524	614,709
Tier I Capital (to Risk Weighted Assets)	873,897	565,148
Total Capital (to Risk Weighted Assets)	1,092,371	941,913
Common Equity Tier I Capital (to Risk Weighted Assets)	$ 710,041
Well Capitalized Regulatory Requirements, Ratio
Tier I Capital (to Average Assets)	5.00%	5.00%
Tier I Capital (to Risk Weighted Assets)	8.00%	6.00%
Total Capital (to Risk Weighted Assets)	10.00%	10.00%
Common Equity Tier I Capital (to Risk Weighted Assets)	6.50%

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common equity capital required to be well capitalized percent

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- Definition

common equity tier 1 capital

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common equity tier 1 to risk weighted assets

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- Definition

The amount of total capital as defined in the regulations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 948

-SubTopic 10

-Section 50

-Paragraph 5

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- Definition

The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

+ References

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Total Capital divided by risk weighted assets as defined in the regulations.

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- Definition

Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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