INCOME TAXES (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2026 USD ($)	Jun. 30, 2025 USD ($)
Income Tax Contingency [Line Items]
Renewable energy investment tax credits, purchased amount	$ 21.4	$ 35.6	$ 66.4	$ 35.6
Renewable energy investment tax credits, price per credit		0.91
Renewable energy investment tax credits, tax benefit	$ 2.0	$ 3.2	$ 6.3	$ 3.2
Minimum
Income Tax Contingency [Line Items]
Renewable energy investment tax credits, price per credit	0.91		0.91
Maximum
Income Tax Contingency [Line Items]
Renewable energy investment tax credits, price per credit	0.93		0.93

Investment Tax Credit, Price Per Credit

No definition available.

Name:	agm_InvestmentTaxCreditPricePerCredit
Namespace Prefix:	agm_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of reported income tax benefit (expense) from difference to income tax expense (benefit) computed by applying statutory federal (national) income tax rate to pretax income (loss) from continuing operation, attributable to energy-related tax credit.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 231
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482663/740-10-55-231

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12A
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-12A

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditEnergyRelatedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_IncomeTaxContingencyLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount deducted from an entities' taxes, reflecting a percentage of an amount invested in certain assets spread over the useful life of those assets; includes deferred investment tax credits.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

+ Details

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

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: