Fair Value Disclosures - Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 [Member] - Fair Value, Measurements, Recurring [Member] - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 4,891,803,000	$ 6,115,339,000	$ 5,472,078,000
Transfers In	7,041,000
Purchases	1,289,876,000	1,825,732,000	1,055,501,000
Sales	(155,744,000)	(124,760,000)	0
Settlements	(533,943,000)	(957,486,000)	(394,307,000)
Transfers Out		(2,013,151,000)
Ending Balance	5,508,576,000	4,891,803,000	6,115,339,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(7,649,000)	(19,478,000)	4,210,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	17,192,000	65,607,000	(22,143,000)
Investment Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	17,730,000	45,415,000	41,265,000
Purchases		0	0
Sales		(26,806,000)	0
Settlements		(2,213,000)	(657,000)
Transfers Out		0
Ending Balance		17,730,000	45,415,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		1,728,000	359,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		(394,000)	4,448,000
USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading Securities, Unrealized Gains	100,000	300,000	900,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	20,388,000	1,917,319,000	1,771,532,000
Transfers In	0
Purchases	155,744,000	391,240,000	363,621,000
Sales	(155,744,000)	(97,954,000)	0
Settlements	(6,849,000)	(245,587,000)	(245,481,000)
Transfers Out		(1,980,327,000)
Ending Balance	13,515,000	20,388,000	1,917,319,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(24,000)	(262,000)	761,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	35,959,000	26,886,000
Available-for-sale Securities [Member] | Investment Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	17,730,000	44,924,000	40,576,000
Transfers In	7,041,000
Purchases	0	0	0
Sales	0	(26,806,000)	0
Settlements	(444,000)	0	0
Transfers Out		0
Ending Balance	23,147,000	17,730,000	44,924,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	6,000	(100,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(1,180,000)	(394,000)	4,448,000
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	4,853,685,000	4,152,605,000	3,659,281,000
Transfers In	0
Purchases	1,134,132,000	1,434,492,000	691,880,000
Sales	0	0	0
Settlements	(526,650,000)	(709,686,000)	(148,169,000)
Transfers Out	0	(32,824,000)	0
Ending Balance	5,471,914,000	4,853,685,000	4,152,605,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(7,625,000)	(20,944,000)	3,090,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	18,372,000	30,042,000	(53,477,000)
Available-for-sale Securities [Member] | USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0	1,888,344,000	1,731,222,000
Transfers In	0
Purchases	155,744,000	391,240,000	363,621,000
Sales	(155,744,000)	(97,954,000)	0
Settlements	0	(237,262,000)	(233,385,000)
Transfers Out	0	(1,980,327,000)	0
Ending Balance	0	0	1,888,344,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	35,959,000	26,886,000
Trading Securities [Member] | Investment Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0	491,000	689,000
Purchases		0	0
Sales		0	0
Settlements		(2,213,000)	(657,000)
Transfers Out		0
Ending Balance		0	491,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		1,722,000	459,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0	0
Trading Securities [Member] | USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	20,388,000	28,975,000	40,310,000
Transfers In	0
Purchases	0	0	0
Sales	0	0	0
Settlements	(6,849,000)	(8,325,000)	(12,096,000)
Transfers Out		0
Ending Balance	13,515,000	20,388,000	28,975,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(24,000)	(262,000)	761,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0	0
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	4,853,685,000	4,121,244,000	3,631,662,000
Transfers In	0
Purchases	1,134,132,000	1,430,392,000	678,566,000
Sales	0	0	0
Settlements	(526,650,000)	(706,446,000)	(138,687,000)
Transfers Out		0
Ending Balance	5,471,914,000	4,853,685,000	4,121,244,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(7,625,000)	(20,944,000)	3,090,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	18,372,000	29,439,000	(53,387,000)
USDA Guarantees [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	0	31,361,000	27,619,000
Purchases		4,100,000	13,314,000
Sales		0	0
Settlements		(3,240,000)	(9,482,000)
Transfers Out		(32,824,000)
Ending Balance		0	31,361,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		603,000	(90,000)
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	17,730,000	44,924,000	40,576,000
Transfers In	0
Purchases	0	0	0
Sales	0	(26,806,000)	0
Settlements	0	0	0
Transfers Out		0
Ending Balance	18,814,000	17,730,000	44,924,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	6,000	(100,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	1,084,000	(394,000)	4,448,000
Floating Interest Rate [Member] | Trading Securities [Member] | Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0	491,000	689,000
Purchases		0	0
Sales		0	0
Settlements		(2,213,000)	(657,000)
Transfers Out		0
Ending Balance		0	491,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		1,722,000	459,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0	$ 0
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0
Transfers In	7,041,000
Purchases	0
Sales	0
Settlements	(444,000)
Ending Balance	4,333,000	$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	$ (2,264,000)