Fair Value Disclosures - Unobservable Input Reconciliation (Details) - Level 3 - Recurring - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 6,477,184	$ 6,901,703	$ 6,358,451	$ 6,973,567
Purchases	1,370,000	0	2,722,750	578,115
Sales	0	0	0	0
Settlements	(628,211)	(709,176)	(1,440,110)	(1,264,981)
Allowance for Losses	0	(64)	(331)	3
Realized and unrealized losses included in Income	(205,206)	(31,425)	(565,190)	(150,303)
Unrealized gains/(losses) included in Other Comprehensive Income	(13,979)	2,014	(75,782)	26,651
Ending Balance	6,999,788	6,163,052	6,999,788	6,163,052
Floating rate auction-rate certificates backed by Government guaranteed student loans \| Floating Interest Rate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	19,061	19,248	19,254	19,171
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Allowance for Losses	11	6	14	(16)
Realized and unrealized losses included in Income	0	0	0	0
Unrealized gains/(losses) included in Other Comprehensive Income	171	246	(25)	345
Ending Balance	19,243	19,500	19,243	19,500
Available-for-sale securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	19,061	19,248	19,254	19,171
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Allowance for Losses	11	6	14	(16)
Realized and unrealized losses included in Income	0	0	0	0
Unrealized gains/(losses) included in Other Comprehensive Income	171	246	(25)	345
Ending Balance	19,243	19,500	19,243	19,500
Farmer Mac Guaranteed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	6,450,212	6,877,405	6,328,559	6,947,701
Purchases	1,370,000	0	2,722,750	578,115
Sales	0	0	0	0
Settlements	(627,599)	(708,882)	(1,436,963)	(1,263,117)
Allowance for Losses	(11)	(70)	(345)	19
Realized and unrealized losses included in Income	(204,450)	(31,462)	(564,242)	(150,265)
Unrealized gains/(losses) included in Other Comprehensive Income	(14,150)	1,768	(75,757)	26,306
Ending Balance	6,974,002	6,138,759	6,974,002	6,138,759
Farmer Mac Guaranteed Securities \| AgVantage Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	6,440,396	6,877,405	6,316,145	6,947,701
Purchases	1,370,000	0	2,722,750	578,115
Sales	0	0	0	0
Settlements	(627,131)	(708,882)	(1,435,758)	(1,263,117)
Allowance for Losses	(11)	(70)	(345)	19
Realized and unrealized losses included in Income	(204,450)	(31,462)	(564,242)	(150,265)
Unrealized gains/(losses) included in Other Comprehensive Income	(13,043)	1,768	(72,789)	26,306
Ending Balance	6,965,761	6,138,759	6,965,761	6,138,759
Farmer Mac Guaranteed Securities \| Farmer Mac Guaranteed USDA Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	9,816		12,414
Purchases	0		0
Sales	0		0
Settlements	(468)		(1,205)
Allowance for Losses	0		0
Realized and unrealized losses included in Income	0		0
Unrealized gains/(losses) included in Other Comprehensive Income	(1,107)		(2,968)
Ending Balance	8,241		8,241
USDA Securities:
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	2,275	5,050	4,401	6,695
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	(383)	(294)	(2,475)	(1,864)
Allowance for Losses	0	0	0	0
Realized and unrealized losses included in Income	(41)	37	(75)	(38)
Unrealized gains/(losses) included in Other Comprehensive Income	0	0	0	0
Ending Balance	1,851	$ 4,793	1,851	$ 4,793
Guarantee and commitment obligations:
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5,636		6,237
Purchases	0		0
Sales	0		0
Settlements	(229)		(672)
Allowance for Losses	0		0
Realized and unrealized losses included in Income	(715)		(873)
Unrealized gains/(losses) included in Other Comprehensive Income	0		0
Ending Balance	4,692		4,692
Guarantee and commitment obligations: \| Guarantee Asset
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5,636		6,237
Purchases	0		0
Sales	0		0
Settlements	(229)		(672)
Allowance for Losses	0		0
Realized and unrealized losses included in Income	(715)		(873)
Unrealized gains/(losses) included in Other Comprehensive Income	0		0
Ending Balance	$ 4,692		$ 4,692

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Allowance for Credit Loss

No definition available.

Name:	agm_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAllowanceForCreditLoss
Namespace Prefix:	agm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ 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 (c)(2)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ 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 (c)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

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

- 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_AuctionRateSecuritiesMember
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_FairValueByAssetClassAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
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_FarmerMacGuaranteedSecuritiesMember
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_FairValueByAssetClassAxis=agm_GuaranteeAndCommitmentObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=agm_GuaranteeAssetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: