Fair Value Disclosures - Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Gains on trading securities		$ 50	$ 326	$ 81
Level 3 | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		7,170,850	6,003,211	5,508,576
Cumulative Effect from Change in Hedge Accounting				487
Purchases	$ 2,091,566	974,237		2,305,396
Sales	(57,853)	0		(127,850)
Settlements	(1,021,706)	(1,400,130)		(1,676,136)
Allowance for Losses		(345)
Realized and unrealized gains included in Income	181,470	202,757		19,448
Unrealized gains included in Other Comprehensive Income	(25,838)	26,198		(26,710)
Ending Balance	7,170,850	6,973,567	7,170,850	6,003,211
Floating rate auction-rate certificates backed by Government guaranteed student loans | Floating Interest Rate | Level 3 | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		18,912
Ending Balance	18,912	19,171	18,912
Floating rate auction-rate certificates backed by Government guaranteed student loans | Floating Interest Rate | Level 3 | Recurring | Available-for-Sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		18,912	18,715	18,814
Cumulative Effect from Change in Hedge Accounting				0
Purchases	0	0		0
Sales	0	0		0
Settlements	0	0		0
Allowance for Losses		(36)
Realized and unrealized gains included in Income	0	0		0
Unrealized gains included in Other Comprehensive Income	197	295		(99)
Ending Balance	18,912	19,171	18,912	18,715
Fixed rate GSE guaranteed mortgage-backed securities | Fixed Interest Rate | Level 3 | Recurring | Available-for-Sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance			0	4,333
Cumulative Effect from Change in Hedge Accounting				0
Purchases				0
Sales				0
Settlements				(2,137)
Realized and unrealized gains included in Income				(2,092)
Unrealized gains included in Other Comprehensive Income				(104)
Ending Balance				0
Available-for-sale securities | Level 3 | Recurring | Available-for-Sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		18,912	18,715	23,147
Cumulative Effect from Change in Hedge Accounting				0
Purchases	0	0		0
Sales	0	0		0
Settlements	0	0		(2,137)
Allowance for Losses		(36)
Realized and unrealized gains included in Income	0	0		(2,092)
Unrealized gains included in Other Comprehensive Income	197	295		(203)
Ending Balance	18,912	19,171	18,912	18,715
Farmer Mac Guaranteed Securities | Level 3 | Recurring | AgVantage Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		7,143,025
Ending Balance	7,143,025	6,947,701	7,143,025
Farmer Mac Guaranteed Securities | Level 3 | Recurring | Available-for-Sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		7,143,025	5,974,497	5,471,914
Cumulative Effect from Change in Hedge Accounting				487
Purchases	2,033,713	974,237		2,177,546
Sales	0	0		0
Settlements	(1,020,294)	(1,397,861)		(1,670,402)
Allowance for Losses		(309)
Realized and unrealized gains included in Income	181,144	202,706		21,459
Unrealized gains included in Other Comprehensive Income	(26,035)	25,903		(26,507)
Ending Balance	7,143,025	6,947,701	7,143,025	5,974,497
Farmer Mac Guaranteed Securities | Level 3 | Recurring | Available-for-Sale Securities | AgVantage Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		7,143,025	5,974,497	5,471,914
Cumulative Effect from Change in Hedge Accounting				487
Purchases	2,033,713	974,237		2,177,546
Sales	0	0		0
Settlements	(1,020,294)	(1,397,861)		(1,670,402)
Allowance for Losses		(309)
Realized and unrealized gains included in Income	181,144	202,706		21,459
Unrealized gains included in Other Comprehensive Income	(26,035)	25,903		(26,507)
Ending Balance	7,143,025	6,947,701	7,143,025	5,974,497
USDA Securities: | Level 3 | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		8,913	9,999	13,515
Cumulative Effect from Change in Hedge Accounting				0
Purchases	57,853	0		127,850
Sales	(57,853)	0		(127,850)
Settlements	(1,412)	(2,269)		(3,597)
Allowance for Losses
Realized and unrealized gains included in Income	326	51		81
Unrealized gains included in Other Comprehensive Income	0	0		0
Ending Balance	8,913	6,695	8,913	9,999
USDA Securities: | Level 3 | Recurring | Available-for-Sale 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
Purchases	57,853			127,850
Sales	(57,853)			(127,850)
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	0
USDA Securities: | Level 3 | Recurring | Trading
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		8,913	9,999	13,515
Cumulative Effect from Change in Hedge Accounting				0
Purchases	0	0		0
Sales	0	0		0
Settlements	(1,412)	(2,269)		(3,597)
Allowance for Losses		0
Realized and unrealized gains included in Income	326	51		81
Unrealized gains included in Other Comprehensive Income	0	0		0
Ending Balance	$ 8,913	$ 6,695	$ 8,913	9,999
Gains on trading securities				$ 100