Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Schedule of Available-for-sale Securities (Details) (Farmer Mac Guaranteed Securities and USDA Securities [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Farmer Mac Guaranteed Securities and USDA Securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 109,217
Available-for-sale Securities, Due after one year through five years, Amortized Cost	1,686,323
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	1,449,257
Available-for-sale Securities, Due after ten years, Amortized Cost	2,128,640
Available-for-sale Securities, Amortized Cost Basis	5,373,437	6,682,569
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	106,395
Available-for-sale Securities, Due after one year through five years, Fair Value	1,708,230
Available-for-sale Securities, Due after five years through ten years, Fair Value	1,455,096
Available-for-sale Securities, Due after ten years, Fair Value	2,120,782
Available-for-sale, at fair value	$ 5,390,503	$ 6,645,269
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	0.93%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	1.84%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	1.72%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	2.57%
Available-for-sale Securities, Total Weighted-Average Yield	2.07%

Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Schedule of Available-for-sale Securities (Details) (Farmer Mac Guaranteed Securities and USDA Securities [Member], USD $)
In Thousands, unless otherwise specified

Dec. 31, 2014

Dec. 31, 2013

Farmer Mac Guaranteed Securities and USDA Securities [Member]

Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]

Available-for-sale Securities, Due within one year, Amortized Cost

$ 109,217us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Due after one year through five years, Amortized Cost

1,686,323us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Due after five years through ten years, Amortized Cost

1,449,257us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Due after ten years, Amortized Cost

2,128,640us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Amortized Cost Basis

5,373,437us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

6,682,569us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]

Available-for-sale Securities, Due within one year, Fair Value

106,395us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Due after one year through five years, Fair Value

1,708,230us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Due after five years through ten years, Fair Value

1,455,096us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Due after ten years, Fair Value

2,120,782us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale, at fair value

$ 5,390,503us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

$ 6,645,269us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]

Available-for-sale Securities, Due within one year, Weighted-Average Yield

0.93%agm_AvailableForSaleSecuritiesDebtMaturitiesNextTwelveMonthsWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield

1.84%agm_AvailableForSaleSecuritiesDebtMaturitiesYearTwoThroughFiveWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield

1.72%agm_AvailableForSaleSecuritiesDebtMaturitiesYearSixThroughTenWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Due after ten years, Weighted-Average Yield

2.57%agm_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Total Weighted-Average Yield

2.07%agm_AvailableForSaleSecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember