Fair Value Disclosures - Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 [Member] - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 6,003,211	$ 5,508,576
Cumulative Effect from Change in Hedge Accounting		487
Purchases	795,260	689,740
Sales	(18,928)	(34,293)
Settlements	(372,289)	(292,350)
Realized and Unrealized Gains included in Income	59,396	(18,932)
Unrealized Gains included in Other Comprehensive Income	3,373	20,847
Ending Balance	6,470,023	5,874,075
USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	9,999	13,515
Cumulative Effect from Change in Hedge Accounting		0
Purchases	18,928	34,293
Sales	(18,928)	(34,293)
Settlements	(556)	(1,973)
Realized and Unrealized Gains included in Income	44	16
Unrealized Gains included in Other Comprehensive Income	0	0
Ending Balance	9,487	11,558
Unrealized Gains		100
Available-for-sale Securities [Member] \| Investment Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	18,715	23,147
Cumulative Effect from Change in Hedge Accounting		0
Purchases	0	0
Sales	0	0
Settlements	0	(109)
Realized and Unrealized Gains included in Income	0	0
Unrealized Gains included in Other Comprehensive Income	197	92
Ending Balance	18,912	23,130
Available-for-sale Securities [Member] \| Farmer Mac Guaranteed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5,974,497	5,471,914
Cumulative Effect from Change in Hedge Accounting		487
Purchases	776,332	655,447
Sales	0	0
Settlements	(371,733)	(290,268)
Realized and Unrealized Gains included in Income	59,352	(18,948)
Unrealized Gains included in Other Comprehensive Income	3,176	20,755
Ending Balance	6,441,624	5,839,387
Available-for-sale Securities [Member] \| USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0	0
Cumulative Effect from Change in Hedge Accounting		0
Purchases	18,928	34,293
Sales	(18,928)	(34,293)
Settlements	0	0
Realized and Unrealized Gains included in Income	0	0
Unrealized Gains included in Other Comprehensive Income	0	0
Ending Balance	0	0
Trading Securities [Member] \| USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	9,999	13,515
Cumulative Effect from Change in Hedge Accounting		0
Purchases	0	0
Sales	0	0
Settlements	(556)	(1,973)
Realized and Unrealized Gains included in Income	44	16
Unrealized Gains included in Other Comprehensive Income	0	0
Ending Balance	9,487	11,558
Institutional Credit [Member] \| Available-for-sale Securities [Member] \| Farmer Mac Guaranteed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5,974,497	5,471,914
Cumulative Effect from Change in Hedge Accounting		487
Purchases	776,332	655,447
Sales	0	0
Settlements	(371,733)	(290,268)
Realized and Unrealized Gains included in Income	59,352	(18,948)
Unrealized Gains included in Other Comprehensive Income	3,176	20,755
Ending Balance	6,441,624	5,839,387
Floating Interest Rate [Member] \| Available-for-sale Securities [Member] \| 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
Cumulative Effect from Change in Hedge Accounting		0
Purchases	0	0
Sales	0	0
Settlements	0	0
Realized and Unrealized Gains included in Income	0	0
Unrealized Gains included in Other Comprehensive Income	197	196
Ending Balance	$ 18,912	19,010
Fixed Interest Rate [Member] \| Available-for-sale Securities [Member] \| Collateralized Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		4,333
Cumulative Effect from Change in Hedge Accounting		0
Purchases		0
Sales		0
Settlements		(109)
Realized and Unrealized Gains included in Income		0
Unrealized Gains included in Other Comprehensive Income		(104)
Ending Balance		$ 4,120

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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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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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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-Publisher FASB
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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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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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