Related Party Transactions - Narrative (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Commitment fees receivable	$ 100,000	$ 100,000
Zions Bancorporation, National Association
Related Party Transaction [Line Items]
Related party percent of Farmer Mac's outstanding business volume	4.10%	4.50%
Servicing fees	$ 11,800,000	$ 12,200,000	$ 11,600,000
CFC
Related Party Transaction [Line Items]
Transaction with related party, percent of volume	9.50%	12.50%	19.10%
Related party percent of Farmer Mac's outstanding business volume	19.20%	21.20%
Servicing fees	$ 3,300,000	$ 3,200,000	$ 3,600,000
Interest receivables	5,300,000	9,200,000
Interest income	63,100,000	97,300,000	76,800,000
Commitment fees earned	1,300,000	1,700,000	1,900,000
Unpaid principal balance	542,443,000	660,000,000	986,645,000
CoBank
Related Party Transaction [Line Items]
Servicing fees	2,300,000	1,200,000
AgFirst Farm Credit Bank
Related Party Transaction [Line Items]
Commitment fees receivable		100,000	100,000
Commitment fees earned	1,200,000
Guarantee fees	25,000	29,000	33,000
AgFirst Farm Credit Bank \| Guarantee Obligations
Related Party Transaction [Line Items]
Unpaid principal balance	32,500,000	26,700,000	26,600,000
Outstanding balance	331,200,000	332,400,000
AgFirst Farm Credit Bank \| Performance Guarantee
Related Party Transaction [Line Items]
Outstanding balance	5,500,000	7,000,000.0
Farm Credit Bank of Texas
Related Party Transaction [Line Items]
Servicing fees	100,000	100,000	200,000
Commitment fees earned	1,200,000	1,100,000	$ 1,000,000.0
Farm Credit Bank of Texas \| Guarantee Obligations
Related Party Transaction [Line Items]
Outstanding balance	$ 304,900,000	$ 270,300,000
Class A voting common stock \| Zions Bancorporation, National Association
Related Party Transaction [Line Items]
Ownership by a related party	31.20%
Class A voting common stock \| CFC
Related Party Transaction [Line Items]
Ownership by a related party	7.90%
Common Class B \| CoBank
Related Party Transaction [Line Items]
Ownership by a related party	32.60%
Common Class B \| AgFirst Farm Credit Bank
Related Party Transaction [Line Items]
Ownership by a related party	16.80%
Common Class B \| Farm Credit Bank of Texas
Related Party Transaction [Line Items]
Ownership by a related party	7.70%
Farm & Ranch \| Zions Bancorporation, National Association
Related Party Transaction [Line Items]
Transaction with related party, percent of volume of loan purchases	7.10%	9.50%	11.90%
Transaction with related party, percent of volume	6.20%	7.60%	8.20%
USDA Guarantee \| Zions Bancorporation, National Association
Related Party Transaction [Line Items]
Transaction with related party, percent of volume	1.40%	2.10%	4.20%
Rural Utilities: \| CFC
Related Party Transaction [Line Items]
Transaction with related party, percent of volume of loan purchases	36.70%
Rural Utilities: \| CoBank
Related Party Transaction [Line Items]
Transaction with related party, percent of volume of loan purchases	56.00%	89.10%
AgVantage Securities \| CFC
Related Party Transaction [Line Items]
Transaction with related party, percent of volume	19.20%	25.50%	29.50%
AgVantage Securities \| CFC \| Guarantee Obligations
Related Party Transaction [Line Items]
Unpaid principal balance	$ 250,000,000	$ 575,000,000	$ 675,000,000
Mortgages \| Farm & Ranch \| Zions Bancorporation, National Association
Related Party Transaction [Line Items]
Unpaid principal balance	177,143,000	129,040,000	114,719,000
Mortgages \| Farm & Ranch \| Farm Credit of Florida [Member]
Related Party Transaction [Line Items]
Unpaid principal balance	200,000	0	0
Mortgages \| Rural Utilities:
Related Party Transaction [Line Items]
Unpaid principal balance	416,800,000	776,400,000
USDA Securities: \| USDA Guarantee \| Zions Bancorporation, National Association
Related Party Transaction [Line Items]
Unpaid principal balance	10,764,000	8,875,000	19,120,000
USDA Securities: \| USDA Guarantee \| Bath State Bank
Related Party Transaction [Line Items]
Unpaid principal balance	$ 9,200,000	$ 4,000,000.0	$ 2,000,000.0

Common Stock Ownership Percentage by a Related Party

No definition available.

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

- Definition

Related Party Transaction, Related Party's percent of total Program

+ References

No definition available.

+ Details

Name:	agm_RelatedPartyTransactionRelatedPartysPercentOfTotalProgram
Namespace Prefix:	agm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Related Party Transaction, Related Party's Percent of Total Program Loans

+ References

No definition available.

+ Details

Name:	agm_RelatedPartyTransactionRelatedPartysPercentOfTotalProgramLoans
Namespace Prefix:	agm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Related Party Transactions, Outstanding Balance

+ References

No definition available.

+ Details

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

- Definition

Related Party's percent of Farmer Mac Total Outstanding Book of Business

+ References

No definition available.

+ Details

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

- Definition

Amount of receivables arising from transactions with related parties due within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3(a)(2))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39603-107864

+ Details

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

- Definition

For an unclassified balance sheet, amounts due from related parties including affiliates, employees, joint ventures, officers and stockholders, immediate families thereof, and pension funds.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(3))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.3)
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

+ Details

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

- Definition

Amount of interest income earned from interest-bearing assets owed to the entity by related party.

+ References

No definition available.

+ Details

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

- Definition

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Reflects the sum of all other revenue and income realized from sales and other transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period.

+ References

No definition available.

+ Details

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

- Definition

Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

No definition available.

+ Details

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

- Definition

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=120401555&loc=SL114874292-224272

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=agm_ZionsBancorporationNationalAssociationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=agm_CfcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=agm_CoBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=agm_AgfirstFarmCreditBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=agm_FarmCreditBankOfTexasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassBMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=agm_RuralUtilitiesMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=agm_FarmCreditOfFloridaMember
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_RelatedPartyTransactionsByRelatedPartyAxis=agm_BathStateBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: