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Investment Securities - Schedule of Investment Securities (Details) (USD $)
 12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2014
Dec. 31, 2013
Available-for-sale Securities [Abstract]          
Derivative, Notional Amount $ 6,837,571,000invest_DerivativeNotionalAmount $ 6,691,851,000invest_DerivativeNotionalAmount   $ 6,837,571,000invest_DerivativeNotionalAmount $ 6,691,851,000invest_DerivativeNotionalAmount
Investment Securities [Member]          
Available-for-sale Securities [Abstract]          
Available-for-Sale Securities Unpaid Principal Balance 1,932,236,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,475,407,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   1,932,236,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,475,407,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Unamortized Premium/(Discount) 7,471,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 10,014,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   7,471,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 10,014,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Available-for-sale Securities, Amortized Cost Basis 1,939,707,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,485,421,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   1,939,707,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,485,421,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 9,003,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 13,713,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   9,003,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 13,713,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Available-for-sale Securities, Unrealized Losses (10,211,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 (15,987,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   (10,211,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 (15,987,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Available-for-sale Securities, Fair Value Disclosure 1,938,499,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,483,147,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   1,938,499,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,483,147,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Proceeds from sale of available-for-sale investment securities 770,149,000us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 366,562,000us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 7,018,000us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		    
Available-for-sale Securities, Gross Realized Gains 600,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 3,100,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 28,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		    
Trading Securities [Abstract]          
Trading, at fair value 689,000us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 928,000us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   689,000us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 928,000us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Marketable Securities [Abstract]          
Marketable Securities Unpaid Principal Balance 1,935,104,000agm_MarketableSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,478,960,000agm_MarketableSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   1,935,104,000agm_MarketableSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,478,960,000agm_MarketableSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Marketable Securities, Amortized Cost 1,942,575,000agm_MarketableSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,488,974,000agm_MarketableSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   1,942,575,000agm_MarketableSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,488,974,000agm_MarketableSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Marketable Securities, Unrealized Gains 9,003,000agm_MarketableSecuritiesGrossUnrealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 13,713,000agm_MarketableSecuritiesGrossUnrealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   9,003,000agm_MarketableSecuritiesGrossUnrealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 13,713,000agm_MarketableSecuritiesGrossUnrealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Marketable Securities, Unrealized Losses (12,390,000)agm_MarketableSecuritiesGrossUnrealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 (18,612,000)agm_MarketableSecuritiesGrossUnrealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   (12,390,000)agm_MarketableSecuritiesGrossUnrealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 (18,612,000)agm_MarketableSecuritiesGrossUnrealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Marketable Securities 1,939,188,000us-gaap_MarketableSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,484,075,000us-gaap_MarketableSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   1,939,188,000us-gaap_MarketableSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 2,484,075,000us-gaap_MarketableSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
USDA Securities [Member]
          
Available-for-sale Securities [Abstract]          
Available-for-sale Securities, Fair Value Disclosure 1,731,222,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 1,553,669,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		   1,731,222,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 1,553,669,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Trading Securities [Abstract]          
Trading, at fair value 40,310,000us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 58,344,000us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		   40,310,000us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 58,344,000us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Marketable Securities [Abstract]          
Marketable Securities 1,771,532,000us-gaap_MarketableSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 1,612,013,000us-gaap_MarketableSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		   1,771,532,000us-gaap_MarketableSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 1,612,013,000us-gaap_MarketableSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Available-for-sale Securities [Member] | Investment Securities [Member]          
Available-for-sale Securities [Abstract]          
Unamortized Premium/(Discount) 7,471,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_AvailableforsaleSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 10,014,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_AvailableforsaleSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   7,471,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_AvailableforsaleSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		 10,014,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_AvailableforsaleSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Floating Interest Rate [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans          
Available-for-sale Securities [Abstract]          
Available-for-Sale Securities Unpaid Principal Balance 46,600,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 74,100,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   46,600,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 74,100,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember
Available-for-sale Securities, Amortized Cost Basis 46,600,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 74,100,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   46,600,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 74,100,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember
Available-for-sale Securities, Unrealized Losses (6,024,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 (8,815,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   (6,024,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 (8,815,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember
Available-for-sale Securities, Fair Value Disclosure 40,576,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 65,285,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   40,576,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 65,285,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AuctionRateSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember
Floating Interest Rate [Member] | Asset-backed Securities [Member]          
Available-for-sale Securities [Abstract]          
Available-for-Sale Securities Unpaid Principal Balance 100,730,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 166,185,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   100,730,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 166,185,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember
Available-for-sale Securities, Amortized Cost Basis 100,656,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 165,968,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   100,656,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 165,968,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 283,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 195,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   283,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 195,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember
Available-for-sale Securities, Unrealized Losses (37,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 (59,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   (37,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 (59,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember
Available-for-sale Securities, Fair Value Disclosure 100,902,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 166,104,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   100,902,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 166,104,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember
Trading Securities [Abstract]          
Trading Securities - Amount Outstanding 2,868,000agm_TradingSecuritiesAmountOutstanding
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		 3,553,000agm_TradingSecuritiesAmountOutstanding
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FloatingInterestRateMember		   2,868,000agm_TradingSecuritiesAmountOutstanding
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_ProductOrServiceAxis
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Available-for-sale Securities [Abstract]          
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Available-for-sale Securities [Abstract]          
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Available-for-sale Securities [Abstract]          
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Available-for-sale Securities [Abstract]          
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Available-for-sale Securities [Abstract]          
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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= agm_FixedInterestRateMember
Fixed Interest Rate [Member] | USDA Securities [Member]          
Available-for-sale Securities [Abstract]          
Available-for-Sale Securities Unpaid Principal Balance 18,806,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
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Fixed Interest Rate [Member] | US Treasury Securities [Member]          
Available-for-sale Securities [Abstract]          
Available-for-Sale Securities Unpaid Principal Balance 975,194,000agm_AvailableforSaleSecuritiesUnpaidPrincipalBalance
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_ProductOrServiceAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
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= us-gaap_USTreasurySecuritiesMember
/ us-gaap_ProductOrServiceAxis
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Available-for-sale Securities, Unrealized Losses (16,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_ProductOrServiceAxis
= agm_FixedInterestRateMember		   (16,000)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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/ us-gaap_ProductOrServiceAxis
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Available-for-sale Securities, Fair Value Disclosure 975,712,000us-gaap_AvailableForSaleSecurities
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/ us-gaap_ProductOrServiceAxis
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/ us-gaap_ProductOrServiceAxis
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Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Corporate Debt Securities [Member]          
Available-for-sale Securities [Abstract]          
Unamortized Premium/(Discount) (10,000)us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]          
Available-for-sale Securities [Abstract]          
Unamortized Premium/(Discount) 3,542,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
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/ us-gaap_ProductOrServiceAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | GSE Preferred Stock [Member]          
Available-for-sale Securities [Abstract]          
Unamortized Premium/(Discount)   365,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
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= us-gaap_AvailableforsaleSecuritiesMember
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= agm_FixedInterestRateMember
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | USDA Securities [Member]          
Available-for-sale Securities [Abstract]          
Unamortized Premium/(Discount) 130,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_ProductOrServiceAxis
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= us-gaap_AvailableforsaleSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
/ us-gaap_ProductOrServiceAxis
= agm_FixedInterestRateMember		   130,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | US Treasury Securities [Member]          
Available-for-sale Securities [Abstract]          
Unamortized Premium/(Discount) 462,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
/ us-gaap_ProductOrServiceAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_ProductOrServiceAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Interest-Only-Strip [Member]          
Available-for-sale Securities [Abstract]          
Unamortized Premium/(Discount) $ 7,300,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_AvailableforsaleSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_ProductOrServiceAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestOnlyStripMember
/ us-gaap_ProductOrServiceAxis
= agm_FixedInterestRateMember		   $ 7,300,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
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= us-gaap_AvailableforsaleSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestOnlyStripMember
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/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_AvailableforsaleSecuritiesMember
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[1] Fair value includes $7.4 million of an interest-only security with a notional amount of $152.4 million.
[2] (1)Fair value includes $7.3 million of an interest-only security with a notional amount of $152.4 million.