Fair Value Disclosures - Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 [Member] - Fair Value, Measurements, Recurring [Member] - USD ($)		3 Months Ended				9 Months Ended
		Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		$ 5,322,025,000		$ 6,741,588,000		$ 4,891,803,000		$ 6,115,339,000
Transfers In						7,041,000
Purchases		234,644,000		382,397,000		1,054,554,000		1,698,586,000
Sales		(40,844,000)		(31,866,000)		(126,939,000)		(85,434,000)
Settlements		(31,392,000)		(123,932,000)		(362,780,000)		(842,537,000)
Realized and Unrealized Gains/(Losses) included in Income		(1,171,000)		(9,778,000)		5,082,000		28,415,000
Unrealized Gains/(Losses) included in Other Comprehen-sive Income		(1,676,000)		470,000		12,825,000		44,510,000
Ending Balance		5,481,586,000		6,958,879,000		5,481,586,000		6,958,879,000
Investment Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance				18,011,000				45,415,000
Purchases				0				0
Sales				0				(26,806,000)
Settlements				(1,887,000)				(2,213,000)
Realized and Unrealized Gains/(Losses) included in Income				1,606,000				1,728,000
Unrealized Gains/(Losses) included in Other Comprehen-sive Income				(148,000)				(542,000)
Ending Balance				17,582,000				17,582,000
USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading Securities, Unrealized Gains		0		400,000		42,000		100,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		16,294,000		1,992,546,000		20,388,000		1,917,319,000
Transfers In						0
Purchases		40,844,000		119,201,000		126,939,000		351,914,000
Sales		(40,844,000)		(31,866,000)		(126,939,000)		(58,628,000)
Settlements		(1,430,000)		(56,646,000)		(5,440,000)		(242,960,000)
Realized and Unrealized Gains/(Losses) included in Income		0		(424,000)		(84,000)		212,000
Unrealized Gains/(Losses) included in Other Comprehen-sive Income		0		(18,995,000)		0		35,959,000
Ending Balance		14,864,000		2,003,816,000		14,864,000		2,003,816,000
Available-for-sale Securities [Member] | Investment Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		23,169,000		17,730,000		17,730,000		44,924,000
Transfers In						7,041,000
Purchases		0		0		0		0
Sales		0		0		0		(26,806,000)
Settlements		(111,000)		0		(334,000)		0
Realized and Unrealized Gains/(Losses) included in Income		0		0		0		6,000
Unrealized Gains/(Losses) included in Other Comprehen-sive Income		(82,000)		(148,000)		(1,461,000)		(542,000)
Ending Balance		22,976,000		17,582,000		22,976,000		17,582,000
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		5,282,562,000		4,731,031,000		4,853,685,000		4,152,605,000
Transfers In						0
Purchases		193,800,000		263,196,000		927,615,000		1,346,672,000
Sales		0		0		0		0
Settlements		(29,851,000)		(65,399,000)		(357,006,000)		(597,364,000)
Realized and Unrealized Gains/(Losses) included in Income		(1,171,000)		(10,960,000)		5,166,000		26,475,000
Unrealized Gains/(Losses) included in Other Comprehen-sive Income		(1,594,000)		19,613,000		14,286,000		9,093,000
Ending Balance		5,443,746,000		4,937,481,000		5,443,746,000		4,937,481,000
Available-for-sale Securities [Member] | USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0		1,967,759,000		0		1,888,344,000
Transfers In						0
Purchases		40,844,000		119,201,000		126,939,000		351,914,000
Sales		(40,844,000)		(31,866,000)		(126,939,000)		(58,628,000)
Settlements		0		(55,772,000)		0		(237,262,000)
Realized and Unrealized Gains/(Losses) included in Income		0		0		0		0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income		0		(18,995,000)		0		35,959,000
Ending Balance		0		1,980,327,000		0		1,980,327,000
Trading Securities [Member] | Investment Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance				281,000				491,000
Purchases				0				0
Sales				0				0
Settlements				(1,887,000)				(2,213,000)
Realized and Unrealized Gains/(Losses) included in Income				1,606,000				1,722,000
Unrealized Gains/(Losses) included in Other Comprehen-sive 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		16,294,000		24,787,000		20,388,000	[1]	28,975,000
Transfers In						0
Purchases		0		0		0		0
Sales		0		0		0		0
Settlements		(1,430,000)		(874,000)		(5,440,000)		(5,698,000)
Realized and Unrealized Gains/(Losses) included in Income		0	[1]	(424,000)	[2]	(84,000)	[1]	212,000	[2]
Unrealized Gains/(Losses) included in Other Comprehen-sive Income		0		0		0		0
Ending Balance		14,864,000	[1]	23,489,000		14,864,000	[1]	23,489,000
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		5,282,562,000		4,697,584,000		4,853,685,000	[1]	4,121,244,000
Transfers In						0
Purchases		193,800,000		263,196,000		927,615,000		1,342,572,000
Sales		0		0		0		0
Settlements		(29,851,000)		(64,895,000)		(357,006,000)		(594,124,000)
Realized and Unrealized Gains/(Losses) included in Income		(1,171,000)		(10,960,000)		5,166,000		26,475,000
Unrealized Gains/(Losses) included in Other Comprehen-sive Income		(1,594,000)		19,732,000		14,286,000		8,490,000
Ending Balance		5,443,746,000	[1]	4,904,657,000		5,443,746,000	[1]	4,904,657,000
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				33,447,000				31,361,000
Purchases				0				4,100,000
Sales				0				0
Settlements				(504,000)				(3,240,000)
Realized and Unrealized Gains/(Losses) included in Income				0				0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income				(119,000)				603,000
Ending Balance				32,824,000				32,824,000
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		18,518,000		17,730,000		17,730,000		44,924,000
Transfers In						0
Purchases		0		0		0		0
Sales		0		0		0		(26,806,000)
Settlements		0		0		0		0
Realized and Unrealized Gains/(Losses) included in Income		0		0		0		6,000
Unrealized Gains/(Losses) included in Other Comprehen-sive Income		0		(148,000)		788,000		(542,000)
Ending Balance		18,518,000		17,582,000		18,518,000		17,582,000
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				281,000				491,000
Purchases				0				0
Sales				0				0
Settlements	[3]			(1,887,000)				(2,213,000)
Realized and Unrealized Gains/(Losses) included in Income				1,606,000				1,722,000
Unrealized Gains/(Losses) included in Other Comprehen-sive Income				0				0
Ending Balance				$ 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		4,651,000				0
Transfers In						7,041,000
Purchases		0				0
Sales		0				0
Settlements		(111,000)				(334,000)
Realized and Unrealized Gains/(Losses) included in Income		0				0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income		(82,000)				(2,249,000)
Ending Balance		$ 4,458,000				$ 4,458,000

[1] Includes unrealized gains of $34,000 attributable to assets still held as of September 30, 2017 that are recorded in "Gains/(losses) on trading securities."

[2] Includes unrealized losses of $0.4 million attributable to assets still held as of September 30, 2016 that are recorded in "Gains/(losses) on trading securities."

[3] Unrealized gains are attributable to assets still held as of September 30, 2016 and are recorded in "Gains/(losses) on trading securities."