Fair Value Disclosures - Unobservable Input Reconciliation (Details) - Level 3 - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 7,924,456	$ 7,064,077	$ 7,170,850	$ 6,003,211
Purchases	122,892	349,654	958,368	1,788,097
Sales	0	(9,506)	0	(57,853)
Settlements	(514,561)	(254,900)	(828,290)	(726,116)
Allowance for losses	(95)		(367)
Realized and unrealized gains/(losses) included in Income	(42,091)	84,213	257,424	258,309
Unrealized gains/(losses) included in Other Comprehensive Income	46,939	(22,846)	(20,445)	(54,956)
Ending Balance	7,537,540	7,210,692	7,537,540	7,210,692
Available-for-Sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	18,283	19,208	18,912	18,715
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Allowance for losses	1		(37)
Realized and unrealized gains/(losses) included in Income	0	0	0	0
Unrealized gains/(losses) included in Other Comprehensive Income	788	0	197	493
Ending Balance	19,072	19,208	19,072	19,208
Available-for-Sale Securities \| Farmer Mac Guaranteed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	7,898,387	7,035,668	7,143,025	5,974,497
Purchases	122,892	340,148	958,368	1,730,244
Sales	0	0	0	0
Settlements	(513,864)	(254,593)	(826,380)	(724,906)
Allowance for losses	(96)		(330)
Realized and unrealized gains/(losses) included in Income	(41,832)	84,164	257,597	258,155
Unrealized gains/(losses) included in Other Comprehensive Income	46,151	(22,846)	(20,642)	(55,449)
Ending Balance	7,511,638	7,182,541	7,511,638	7,182,541
USDA Securities:
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance			8,913
Ending Balance	6,830		6,830
AgVantage Securities \| Available-for-Sale Securities \| Farmer Mac Guaranteed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	7,898,387	7,035,668	7,143,025	5,974,497
Purchases	122,892	340,148	958,368	1,730,244
Sales	0	0	0	0
Settlements	(513,864)	(254,593)	(826,380)	(724,906)
Allowance for losses	(96)		(330)
Realized and unrealized gains/(losses) included in Income	(41,832)	84,164	257,597	258,155
Unrealized gains/(losses) included in Other Comprehensive Income	46,151	(22,846)	(20,642)	(55,449)
Ending Balance	7,511,638	7,182,541	7,511,638	7,182,541
USDA Guarantees \| Available-for-Sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0		0
Purchases		9,506		57,853
Sales		(9,506)		(57,853)
Settlements		0		0
Realized and unrealized gains/(losses) included in Income		0		0
Unrealized gains/(losses) included in Other Comprehensive Income		0		0
Ending Balance		0		0
USDA Guarantees \| Trading
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	7,786	9,201	8,913	9,999
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	(697)	(307)	(1,910)	(1,210)
Allowance for losses	0		0
Realized and unrealized gains/(losses) included in Income	(259)	49	(173)	154
Unrealized gains/(losses) included in Other Comprehensive Income	0	0	0	0
Ending Balance	6,830	8,943	6,830	8,943
USDA Guarantees \| USDA Securities:
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	7,786	9,201	8,913	9,999
Purchases	0	9,506	0	57,853
Sales	0	(9,506)	0	(57,853)
Settlements	(697)	(307)	(1,910)	(1,210)
Allowance for losses
Realized and unrealized gains/(losses) included in Income	(259)	49	(173)	154
Unrealized gains/(losses) included in Other Comprehensive Income	0	0	0	0
Ending Balance	6,830	8,943	6,830	8,943
Floating Interest Rate \| Available-for-Sale Securities \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	18,283	19,208	18,912	18,715
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Allowance for losses	1		(37)
Realized and unrealized gains/(losses) included in Income	0	0	0	0
Unrealized gains/(losses) included in Other Comprehensive Income	788	0	197	493
Ending Balance	$ 19,072	$ 19,208	$ 19,072	$ 19,208

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

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

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- 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=117815213&loc=d3e19207-110258

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

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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- 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.

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-Publisher FASB
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-Section 50
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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.

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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.

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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.

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