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	Mar. 31, 2015	Dec. 31, 2014
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	$ 563,745
Available-for-sale Securities, Due after one year through five years, Amortized Cost	1,181,719
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	1,467,700
Available-for-sale Securities, Due after ten years, Amortized Cost	2,337,778
Available-for-sale Securities, Amortized Cost Basis	5,550,942	5,373,437
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	571,986
Available-for-sale Securities, Due after one year through five years, Fair Value	1,201,725
Available-for-sale Securities, Due after five years through ten years, Fair Value	1,497,617
Available-for-sale Securities, Due after ten years, Fair Value	2,365,725
Available-for-sale, at fair value	$ 5,637,053	$ 5,390,503
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	2.78%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	1.35%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	1.75%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	2.44%
Available-for-sale Securities, Total Weighted-Average Yield	2.06%

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

Mar. 31, 2015

Dec. 31, 2014

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

$ 563,745us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

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

1,181,719us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

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

1,467,700us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

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

2,337,778us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale Securities, Amortized Cost Basis

5,550,942us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

5,373,437us-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

571,986us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

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

1,201,725us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

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

1,497,617us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

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

2,365,725us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

Available-for-sale, at fair value

$ 5,637,053us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

$ 5,390,503us-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

2.78%agm_AvailableForSaleSecuritiesDebtMaturitiesNextTwelveMonthsWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

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

1.35%agm_AvailableForSaleSecuritiesDebtMaturitiesYearTwoThroughFiveWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

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

1.75%agm_AvailableForSaleSecuritiesDebtMaturitiesYearSixThroughTenWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

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

2.44%agm_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember

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

2.06%agm_AvailableForSaleSecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember