Fair Value Disclosures - Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 6,003,211	$ 5,508,576	$ 4,891,803
Cumulative Effect from Change in Hedge Accounting		487	7,041
Purchases	2,091,566	2,305,396	1,289,876
Sales	(57,853)	(127,850)	(155,744)
Settlements	(1,021,706)	(1,676,136)	(533,943)
Realized and Unrealized (Losses)/Gains included in Income	181,470	19,448	(7,649)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income	(25,838)	(26,710)	17,192
Ending Balance	7,170,850	6,003,211	5,508,576
USDA Securities:
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	9,999	13,515	20,388
Cumulative Effect from Change in Hedge Accounting		0	0
Purchases	57,853	127,850	155,744
Sales	(57,853)	(127,850)	(155,744)
Settlements	(1,412)	(3,597)	(6,849)
Realized and Unrealized (Losses)/Gains included in Income	326	81	(24)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income	0	0	0
Ending Balance	8,913	9,999	13,515
Unrealized gains attributable to assets held		100	100
Available-for-Sale Securities \| Farmer Mac Guaranteed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5,974,497	5,471,914	4,853,685
Cumulative Effect from Change in Hedge Accounting		487	0
Purchases	2,033,713	2,177,546	1,134,132
Sales	0	0	0
Settlements	(1,020,294)	(1,670,402)	(526,650)
Realized and Unrealized (Losses)/Gains included in Income	181,144	21,459	(7,625)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income	(26,035)	(26,507)	18,372
Ending Balance	7,143,025	5,974,497	5,471,914
Available-for-Sale Securities \| USDA Securities:
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0	0	0
Cumulative Effect from Change in Hedge Accounting		0	0
Purchases	57,853	127,850	155,744
Sales	(57,853)	(127,850)	(155,744)
Settlements	0	0	0
Realized and Unrealized (Losses)/Gains included in Income	0	0	0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income	0	0	0
Ending Balance	0	0	0
Available-for-Sale Securities \| Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	18,715	23,147	17,730
Cumulative Effect from Change in Hedge Accounting		0	7,041
Purchases	0	0	0
Sales	0	0	0
Settlements	0	(2,137)	(444)
Realized and Unrealized (Losses)/Gains included in Income	0	(2,092)	0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income	197	(203)	(1,180)
Ending Balance	18,912	18,715	23,147
Trading Securities \| USDA Securities:
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	9,999	13,515	20,388
Cumulative Effect from Change in Hedge Accounting		0	0
Purchases	0	0	0
Sales	0	0	0
Settlements	(1,412)	(3,597)	(6,849)
Realized and Unrealized (Losses)/Gains included in Income	326	81	(24)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income	0	0	0
Ending Balance	8,913	9,999	13,515
Institutional Credit \| Available-for-Sale Securities \| Farmer Mac Guaranteed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5,974,497	5,471,914	4,853,685
Cumulative Effect from Change in Hedge Accounting		487	0
Purchases	2,033,713	2,177,546	1,134,132
Sales	0	0	0
Settlements	(1,020,294)	(1,670,402)	(526,650)
Realized and Unrealized (Losses)/Gains included in Income	181,144	21,459	(7,625)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income	(26,035)	(26,507)	18,372
Ending Balance	7,143,025	5,974,497	5,471,914
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	18,715	18,814	17,730
Cumulative Effect from Change in Hedge Accounting		0	0
Purchases	0	0	0
Sales	0	0	0
Settlements	0	0	0
Realized and Unrealized (Losses)/Gains included in Income	0	0	0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income	197	(99)	1,084
Ending Balance	18,912	18,715	18,814
Fixed Interest Rate \| Available-for-Sale Securities \| Government/GSE Guaranteed Mortgage Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 0	4,333	0
Cumulative Effect from Change in Hedge Accounting		0	7,041
Purchases		0	0
Sales		0	0
Settlements		(2,137)	(444)
Realized and Unrealized (Losses)/Gains included in Income		(2,092)	0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income		(104)	(2,264)
Ending Balance		$ 0	$ 4,333

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

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

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Amount of gain (loss) recognized in the income statement for 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 gain (loss) recognized in other comprehensive income (loss) for 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 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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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 transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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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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