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Fair Value Disclosures - Narrative (Details) (Level 3 [Member], USD $)
In Billions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Level 3 [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets and Liabilities, Fair Value Disclosure $ 5.7agm_AssetsAndLiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 $ 5.5agm_AssetsAndLiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Fair Value, Assets and Liabilities Fair Value as Percent of Total Assets 39.00%agm_FairValueAssetsAndLiabilitiesFairValueAsPercentOfTotalAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 38.00%agm_FairValueAssetsAndLiabilitiesFairValueAsPercentOfTotalAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Fair Value, Assets and Liabilities, Fair Value as Percent of Total Financial Instruments Measured at Fair Value 72.00%agm_FairValueAssetsAndLiabilitiesFairValueAsPercentOfTotalFinancialInstrumentsMeasuredAtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 73.00%agm_FairValueAssetsAndLiabilitiesFairValueAsPercentOfTotalFinancialInstrumentsMeasuredAtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member