Fair Value Disclosures - Unobservable Input Reconciliation (Details) - Level 3 - Recurring - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 6,787,933	$ 7,612,315	$ 6,973,567	$ 7,170,850
Purchases	417,500	351,896	578,115	835,476
Sales	0	0	0	0
Settlements	(310,870)	(85,863)	(555,805)	(313,729)
Allowance for Losses	(90)	(84)	67	(272)
Realized and unrealized gains/(losses) included in Income	49,878	9,030	(118,878)	299,515
Unrealized gains/(losses) included in Other Comprehensive Income	(42,648)	37,162	24,637	(67,384)
Ending Balance	6,901,703	7,924,456	6,901,703	7,924,456
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			19,171
Ending Balance	19,248		19,248
Floating rate auction-rate certificates backed by Government guaranteed student loans \| Floating Interest Rate \| Available-for-Sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	19,146	16,721	19,171	18,912
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Allowance for Losses	3	(15)	(22)	(38)
Realized and unrealized gains/(losses) included in Income	0	0	0	0
Unrealized gains/(losses) included in Other Comprehensive Income	99	1,577	99	(591)
Ending Balance	19,248	18,283	19,248	18,283
Available-for-sale securities \| Available-for-Sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	19,146	16,721	19,171	18,912
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Allowance for Losses	3	(15)	(22)	(38)
Realized and unrealized gains/(losses) included in Income	0	0	0	0
Unrealized gains/(losses) included in Other Comprehensive Income	99	1,577	99	(591)
Ending Balance	19,248	18,283	19,248	18,283
Farmer Mac Guaranteed Securities \| AgVantage Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance			6,947,701
Ending Balance	6,877,405		6,877,405
Farmer Mac Guaranteed Securities \| Available-for-Sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	6,763,209	7,587,186	6,947,701	7,143,025
Purchases	417,500	351,896	578,115	835,476
Sales	0	0	0	0
Settlements	(310,403)	(85,261)	(554,235)	(312,516)
Allowance for Losses	(93)	(69)	89	(234)
Realized and unrealized gains/(losses) included in Income	49,939	9,050	(118,803)	299,429
Unrealized gains/(losses) included in Other Comprehensive Income	(42,747)	35,585	24,538	(66,793)
Ending Balance	6,877,405	7,898,387	6,877,405	7,898,387
Farmer Mac Guaranteed Securities \| Available-for-Sale Securities \| AgVantage Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	6,763,209	7,587,186	6,947,701	7,143,025
Purchases	417,500	351,896	578,115	835,476
Sales	0	0	0	0
Settlements	(310,403)	(85,261)	(554,235)	(312,516)
Allowance for Losses	(93)	(69)	89	(234)
Realized and unrealized gains/(losses) included in Income	49,939	9,050	(118,803)	299,429
Unrealized gains/(losses) included in Other Comprehensive Income	(42,747)	35,585	24,538	(66,793)
Ending Balance	6,877,405	7,898,387	6,877,405	7,898,387
USDA Securities:
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5,578	8,408	6,695	8,913
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	(467)	(602)	(1,570)	(1,213)
Allowance for Losses
Realized and unrealized gains/(losses) included in Income	(61)	(20)	(75)	86
Unrealized gains/(losses) included in Other Comprehensive Income	0	0	0	0
Ending Balance	5,050	7,786	5,050	7,786
USDA Securities: \| Trading
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5,578	8,408	6,695	8,913
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	(467)	(602)	(1,570)	(1,213)
Allowance for Losses	0	0	0	0
Realized and unrealized gains/(losses) included in Income	(61)	(20)	(75)	86
Unrealized gains/(losses) included in Other Comprehensive Income	0	0	0	0
Ending Balance	$ 5,050	$ 7,786	$ 5,050	$ 7,786

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 http://asc.fasb.org/extlink&oid=123874694&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 http://asc.fasb.org/extlink&oid=123874694&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 http://asc.fasb.org/extlink&oid=123874694&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 http://asc.fasb.org/extlink&oid=123874694&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_InformationByCategoryOfDebtSecurityAxis=us-gaap_AvailableforsaleSecuritiesMember
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_FairValueByAssetClassAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InformationByCategoryOfDebtSecurityAxis=agm_DebtSecuritiesTradingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: