Fair Value Disclosures - Fair Value Measurements, Recurring and Nonrecurring (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Assets:
Financial derivatives	$ 16,123,000	$ 7,834,000
Loans held for sale	0	0
Liabilities:
Financial derivatives	182,371,000	105,087,000
Investment securities:
Assets:
Available-for-sale, at fair value	3,853,692,000	$ 2,959,843,000
Farmer Mac Guaranteed Securities
Assets:
Available-for-sale, at fair value	$ 6,947,701,000
Level 3
Liabilities:
Financial instruments level three percent	65.00%	71.00%
Percent of level three fair value assets and liabilities	29.00%	33.00%
Recurring
Assets:
Financial derivatives	$ 17,468,000	$ 10,519,000
Total Assets at fair value	10,825,556,000	10,122,300,000
Liabilities:
Financial derivatives	29,892,000	27,042,000
Total Liabilities at fair value	29,892,000	27,042,000
Recurring \| USDA Securities:
Assets:
Trading	6,695,000	8,913,000
Recurring \| Investment securities:
Assets:
Available-for-sale, at fair value	3,853,692,000	2,959,843,000
Recurring \| Farmer Mac Guaranteed Securities
Assets:
Available-for-sale, at fair value	6,947,701,000	7,143,025,000
Recurring \| Level 1
Assets:
Financial derivatives	0	0
Total Assets at fair value	1,467,951,000	1,296,923,000
Liabilities:
Financial derivatives	82,000	51,000
Total Liabilities at fair value	82,000	51,000
Recurring \| Level 1 \| USDA Securities:
Assets:
Trading	0	0
Recurring \| Level 1 \| Investment securities:
Assets:
Available-for-sale, at fair value	1,467,951,000	1,296,923,000
Recurring \| Level 1 \| Farmer Mac Guaranteed Securities
Assets:
Available-for-sale, at fair value	0	0
Recurring \| Level 2
Assets:
Financial derivatives	17,468,000	10,519,000
Total Assets at fair value	2,384,038,000	1,654,527,000
Liabilities:
Financial derivatives	29,810,000	26,991,000
Total Liabilities at fair value	29,810,000	26,991,000
Recurring \| Level 2 \| USDA Securities:
Assets:
Trading	0	0
Recurring \| Level 2 \| Investment securities:
Assets:
Available-for-sale, at fair value	2,366,570,000	1,644,008,000
Recurring \| Level 2 \| Farmer Mac Guaranteed Securities
Assets:
Available-for-sale, at fair value	0	0
Recurring \| Level 3
Assets:
Financial derivatives	0	0
Total Assets at fair value	6,973,567,000	7,170,850,000
Liabilities:
Financial derivatives	0	0
Total Liabilities at fair value	0	0
Recurring \| Level 3 \| USDA Securities:
Assets:
Trading	6,695,000	8,913,000
Recurring \| Level 3 \| Investment securities:
Assets:
Available-for-sale, at fair value	19,171,000	18,912,000
Recurring \| Level 3 \| Farmer Mac Guaranteed Securities
Assets:
Available-for-sale, at fair value	6,947,701,000	7,143,025,000
AgVantage Securities \| Farmer Mac Guaranteed Securities
Assets:
Available-for-sale, at fair value	6,947,701,000	7,143,025,000
AgVantage Securities \| Recurring \| Farmer Mac Guaranteed Securities
Assets:
Available-for-sale, at fair value	6,947,701,000	7,143,025,000
AgVantage Securities \| Recurring \| Level 1 \| Farmer Mac Guaranteed Securities
Assets:
Available-for-sale, at fair value	0	0
AgVantage Securities \| Recurring \| Level 2 \| Farmer Mac Guaranteed Securities
Assets:
Available-for-sale, at fair value	0	0
AgVantage Securities \| Recurring \| Level 3 \| Farmer Mac Guaranteed Securities
Assets:
Available-for-sale, at fair value	6,947,701,000	7,143,025,000
USDA Guarantees \| Recurring \| USDA Securities:
Assets:
Trading	6,695,000	8,913,000
USDA Guarantees \| Recurring \| Level 1 \| USDA Securities:
Assets:
Trading	0	0
USDA Guarantees \| Recurring \| Level 2 \| USDA Securities:
Assets:
Trading	0	0
USDA Guarantees \| Recurring \| Level 3 \| USDA Securities:
Assets:
Trading	6,695,000	8,913,000
Floating Interest Rate \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Assets:
Available-for-sale, at fair value	19,171,000	18,912,000
Floating Interest Rate \| Floating rate asset-backed securities
Assets:
Available-for-sale, at fair value	6,231,000	11,085,000
Floating Interest Rate \| Guaranteed mortgage-backed securities
Assets:
Available-for-sale, at fair value	2,360,026,000	1,632,583,000
Floating Interest Rate \| Recurring \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Assets:
Available-for-sale, at fair value	19,171,000	18,912,000
Floating Interest Rate \| Recurring \| Floating rate asset-backed securities
Assets:
Available-for-sale, at fair value	6,231,000	11,085,000
Floating Interest Rate \| Recurring \| Guaranteed mortgage-backed securities
Assets:
Available-for-sale, at fair value	2,360,026,000	1,632,583,000
Floating Interest Rate \| Recurring \| Level 1 \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Assets:
Available-for-sale, at fair value	0	0
Floating Interest Rate \| Recurring \| Level 1 \| Floating rate asset-backed securities
Assets:
Available-for-sale, at fair value	0	0
Floating Interest Rate \| Recurring \| Level 1 \| Guaranteed mortgage-backed securities
Assets:
Available-for-sale, at fair value	0	0
Floating Interest Rate \| Recurring \| Level 2 \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Assets:
Available-for-sale, at fair value	0	0
Floating Interest Rate \| Recurring \| Level 2 \| Floating rate asset-backed securities
Assets:
Available-for-sale, at fair value	6,231,000	11,085,000
Floating Interest Rate \| Recurring \| Level 2 \| Guaranteed mortgage-backed securities
Assets:
Available-for-sale, at fair value	2,360,026,000	1,632,583,000
Floating Interest Rate \| Recurring \| Level 3 \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Assets:
Available-for-sale, at fair value	19,171,000	18,912,000
Floating Interest Rate \| Recurring \| Level 3 \| Floating rate asset-backed securities
Assets:
Available-for-sale, at fair value	0	0
Floating Interest Rate \| Recurring \| Level 3 \| Guaranteed mortgage-backed securities
Assets:
Available-for-sale, at fair value	0	0
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Assets:
Available-for-sale, at fair value	313,000	340,000
Fixed Interest Rate \| Fixed rate U.S. Treasuries
Assets:
Available-for-sale, at fair value	1,467,951,000	1,296,923,000
Fixed Interest Rate \| Recurring \| Guaranteed mortgage-backed securities
Assets:
Available-for-sale, at fair value	313,000	340,000
Fixed Interest Rate \| Recurring \| Fixed rate U.S. Treasuries
Assets:
Available-for-sale, at fair value	1,467,951,000	1,296,923,000
Fixed Interest Rate \| Recurring \| Level 1 \| Guaranteed mortgage-backed securities
Assets:
Available-for-sale, at fair value	0	0
Fixed Interest Rate \| Recurring \| Level 1 \| Fixed rate U.S. Treasuries
Assets:
Available-for-sale, at fair value	1,467,951,000	1,296,923,000
Fixed Interest Rate \| Recurring \| Level 2 \| Guaranteed mortgage-backed securities
Assets:
Available-for-sale, at fair value	313,000	340,000
Fixed Interest Rate \| Recurring \| Level 2 \| Fixed rate U.S. Treasuries
Assets:
Available-for-sale, at fair value	0	0
Fixed Interest Rate \| Recurring \| Level 3 \| Guaranteed mortgage-backed securities
Assets:
Available-for-sale, at fair value	0	0
Fixed Interest Rate \| Recurring \| Level 3 \| Fixed rate U.S. Treasuries
Assets:
Available-for-sale, at fair value	$ 0	$ 0

Fair Value, Assets and Liabilities Fair Value as Percent of Total Assets

No definition available.

Name:	agm_FairValueAssetsAndLiabilitiesFairValueAsPercentOfTotalAssets
Namespace Prefix:	agm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair Value, Assets and Liabilities, Fair Value as Percent of Total Financial Instruments Measured at Fair Value

+ References

No definition available.

+ Details

Name:	agm_FairValueAssetsAndLiabilitiesFairValueAsPercentOfTotalFinancialInstrumentsMeasuredAtFairValue
Namespace Prefix:	agm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_AssetsFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

No definition available.

+ Details

Name:	us-gaap_LoansHeldForSaleFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

+ Details

Name:	us-gaap_TradingSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=agm_InstitutionalCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=agm_USDAGuaranteesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=agm_FloatingInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AuctionRateSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=agm_FixedInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: