Farmer Mac Guaranteed Securities and USDA Securities - Farmer Mac Guaranteed Securities and USDA Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Farmer Mac Guaranteed Securities and USDA Securities [Member]
Held-to-maturity:
Unpaid Principal Balance	$ 3,575,170	$ 4,207,531
Amortized Cost	3,616,680	4,262,792
Unrealized Gains	36,928	2,735
Unrealized Losses	(1,990)	(74,175)
Fair Value	3,651,618	4,191,352
Farmer Mac Guaranteed Securities [Member]
Held-to-maturity:
Unpaid Principal Balance	1,452,665	2,096,568
Amortized Cost	1,452,616	2,096,618
Unrealized Gains	16,645	2,735
Unrealized Losses	(717)	(11,948)
Fair Value	1,468,544	2,087,405
Available-for-sale:
Amortized Cost	7,008,885
Fair Value	7,182,541
USDA Securities [Member]
Trading:
Fair Value	8,943	9,999
Held-to-maturity Securities [Member] \| Farmer Mac Guaranteed Securities and USDA Securities [Member]
Held-to-maturity:
Unamortized Premium/(Discount)	41,510	55,261
Available-for-sale:
Unamortized Premium/(Discount)	41,510	55,261
Trading:
Unamortized Premium/(Discount)	41,510	55,261
Held-to-maturity Securities [Member] \| Farmer Mac Guaranteed Securities [Member]
Held-to-maturity:
Unamortized Premium/(Discount)	(49)	50
Available-for-sale:
Unamortized Premium/(Discount)	(49)	50
Trading:
Unamortized Premium/(Discount)	(49)	50
Institutional Credit [Member] \| Farmer Mac Guaranteed Securities [Member]
Held-to-maturity:
Unpaid Principal Balance	1,420,020	2,069,185
Amortized Cost	1,419,800	2,068,991
Unrealized Gains	16,019	2,637
Unrealized Losses	(660)	(11,948)
Fair Value	1,435,159	2,059,680
Available-for-sale:
Unpaid Principal Balance	7,009,022	6,003,733
Amortized Cost	7,008,885	6,003,529
Unrealized Gains	211,878	22,335
Unrealized Losses	(38,222)	(51,367)
Fair Value	7,182,541	5,974,497
Institutional Credit [Member] \| Held-to-maturity Securities [Member] \| Farmer Mac Guaranteed Securities [Member]
Held-to-maturity:
Unamortized Premium/(Discount)	(220)	(194)
Available-for-sale:
Unamortized Premium/(Discount)	(220)	(194)
Trading:
Unamortized Premium/(Discount)	(220)	(194)
Institutional Credit [Member] \| Available-for-sale Securities [Member] \| Farmer Mac Guaranteed Securities [Member]
Held-to-maturity:
Unamortized Premium/(Discount)	(137)	(204)
Available-for-sale:
Unamortized Premium/(Discount)	(137)	(204)
Trading:
Unamortized Premium/(Discount)	(137)	(204)
USDA Guarantees [Member]
Held-to-maturity:
Unamortized Premium/(Discount)	(200)	(200)
Available-for-sale:
Unamortized Premium/(Discount)	(200)	(200)
Trading:
Unamortized Premium/(Discount)	(200)	(200)
USDA Guarantees [Member] \| Farmer Mac Guaranteed Securities [Member]
Held-to-maturity:
Unpaid Principal Balance	32,645	27,383
Amortized Cost	32,816	27,627
Unrealized Gains	626	98
Unrealized Losses	(57)	0
Fair Value	33,385	27,725
USDA Guarantees [Member] \| USDA Securities [Member]
Held-to-maturity:
Unpaid Principal Balance	2,122,505	2,110,963
Amortized Cost	2,164,064	2,166,174
Unrealized Gains	20,283	0
Unrealized Losses	(1,273)	(62,227)
Fair Value	2,183,074	2,103,947
Trading:
Unpaid Principal Balance	8,561	9,591
Amortized Cost	9,082	10,292
Unrealized Gains	14	20
Unrealized Losses	(153)	(313)
Fair Value	$ 8,943	$ 9,999
Trading securities, weighted-average yield	5.21%	5.21%
USDA Guarantees [Member] \| Held-to-maturity Securities [Member] \| Farmer Mac Guaranteed Securities [Member]
Held-to-maturity:
Unamortized Premium/(Discount)	$ 171	$ 244
Available-for-sale:
Unamortized Premium/(Discount)	171	244
Trading:
Unamortized Premium/(Discount)	171	244
USDA Guarantees [Member] \| Held-to-maturity Securities [Member] \| USDA Securities [Member]
Held-to-maturity:
Unamortized Premium/(Discount)	41,559	55,211
Available-for-sale:
Unamortized Premium/(Discount)	41,559	55,211
Trading:
Unamortized Premium/(Discount)	41,559	55,211
USDA Guarantees [Member] \| Trading Securities [Member] \| USDA Securities [Member]
Held-to-maturity:
Unamortized Premium/(Discount)	521	701
Available-for-sale:
Unamortized Premium/(Discount)	521	701
Trading:
Unamortized Premium/(Discount)	$ 521	$ 701

Debt Securities, Available-for-Sale, Unpaid Principal Balance

No definition available.

Name:	agm_DebtSecuritiesAvailableforSaleUnpaidPrincipalBalance
Namespace Prefix:	agm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Held-to-Maturity Unpaid Principal Balance

+ References

No definition available.

+ Details

Name:	agm_HeldtoMaturityUnpaidPrincipalBalance
Namespace Prefix:	agm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Trading Securities Unpaid Principal Balance

+ References

No definition available.

+ Details

Name:	agm_TradingSecuritiesUnpaidPrincipalBalance
Namespace Prefix:	agm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Trading Securities, Weighted-Average Yield

+ References

No definition available.

+ Details

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

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost 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 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAbstract
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 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: 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=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Amount, after accumulated amortization, of debt discount (premium).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28567-108399

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of unrealized gain on 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 50
-Paragraph 9
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

+ Details

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

- Definition

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

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

+ Details

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

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

+ Details

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

- Definition

Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

+ References

No definition available.

+ Details

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

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesFairValue
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) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amortized cost of investments in debt securities classified as trading.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
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_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=agm_InstitutionalCreditMemberMember
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_StatementBusinessSegmentsAxis=agm_USDAGuaranteesMember
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: