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Fair Value Disclosures - Quantitative Information (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Derivative Liability 38,217us-gaap_DerivativeLiabilities 64,388us-gaap_DerivativeLiabilities
Fair Value, Measurements, Recurring [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Prepayment Rate 10.00%us-gaap_FairValueInputsPrepaymentRate
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Prepayment Rate 6.00%us-gaap_FairValueInputsPrepaymentRate
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Range of Offered Quotes 82.00%agm_FairValueInputsRangeOfOfferedQuotes
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 12.20%us-gaap_FairValueInputsDiscountRate
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 1.70%us-gaap_FairValueInputsDiscountRate
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 0.80%us-gaap_FairValueInputsDiscountRate
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 0.80%us-gaap_FairValueInputsDiscountRate
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 0.60%us-gaap_FairValueInputsDiscountRate
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Range of Offered Quotes 92.00%agm_FairValueInputsRangeOfOfferedQuotes
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ us-gaap_RangeAxis
= us-gaap_MaximumMember
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Maximum [Member] | Cost Approach Valuation Technique [Member] | Collateralized Mortgage Obligations [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 1.70%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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= us-gaap_MaximumMember
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Maximum [Member] | Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 2.20%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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= agm_InstitutionalCreditMemberMember
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Maximum [Member] | Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 3.20%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_StatementBusinessSegmentsAxis
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Maximum [Member] | USDA Guarantees [Member] | Cost Approach Valuation Technique [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 5.20%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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Maximum [Member] | Derivative Financial Instruments, Liabilities [Member] | Cost Approach Valuation Technique [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 2.40%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
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/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_CostApproachValuationTechniqueMember		 2.30%us-gaap_FairValueInputsDiscountRate
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Fair Value Inputs, Prepayment Rate 11.00%us-gaap_FairValueInputsPrepaymentRate
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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Weighted Average [Member] | Market Approach Valuation Technique [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Range of Offered Quotes 87.70%agm_FairValueInputsRangeOfOfferedQuotes
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
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/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember
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= us-gaap_MarketApproachValuationTechniqueMember		 88.10%agm_FairValueInputsRangeOfOfferedQuotes
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AuctionRateSecuritiesMember
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= us-gaap_WeightedAverageMember
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Weighted Average [Member] | Cost Approach Valuation Technique [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 16.60%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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= us-gaap_WeightedAverageMember
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Weighted Average [Member] | Cost Approach Valuation Technique [Member] | Collateralized Mortgage Obligations [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 1.70%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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Weighted Average [Member] | Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 1.50%us-gaap_FairValueInputsDiscountRate
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Weighted Average [Member] | Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 1.80%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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Weighted Average [Member] | USDA Guarantees [Member] | Cost Approach Valuation Technique [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 3.20%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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Weighted Average [Member] | Derivative Financial Instruments, Liabilities [Member] | Cost Approach Valuation Technique [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair Value Inputs, Discount Rate 1.20%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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= us-gaap_WeightedAverageMember
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