Fair Value Disclosures - Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands		3 Months Ended				12 Months Ended
		Sep. 30, 2016	Dec. 31, 2014		Mar. 31, 2014	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					$ 235			$ 0		$ 235
Purchases										0
Sales										0
Settlements										0
Realized and Unrealized Gains Included in Income										235
Unrealized Gains/(Losses) included in Other Comprehensive Income										0
Transfers Out										0
Ending Balance			$ 0							0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					6,770,031	$ 6,115,339		5,472,078		6,770,031
Purchases						1,825,732		1,055,501		1,434,461
Sales						(124,760)		0		(26,675)
Settlements						(957,486)		(394,307)		(1,153,191)
Realized and Unrealized Gains/(Losses) included in Income						(19,478)		4,210		15,292
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						65,607		(22,143)		64,946
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3						2,013,151				1,632,786
Ending Balance			5,472,078			4,891,803		6,115,339		5,472,078
Investment Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					66,418	45,415		41,265		66,418
Purchases						0		0		0
Sales						(26,806)		0		(26,675)
Settlements						(2,213)		(657)		(890)
Realized and Unrealized Gains/(Losses) included in Income						1,728		359		(379)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						(394)		4,448		2,791
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3						0				0
Ending Balance			41,265			17,730		45,415		41,265
USDA Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading Securities, Unrealized Gains		$ 300				1,800		900
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					1,612,013	1,917,319		1,771,532		1,612,013
Purchases						391,240		363,621		335,359
Sales						(97,954)		0		0
Settlements						(245,587)		(245,481)		(228,585)
Realized and Unrealized Gains/(Losses) included in Income						(262)		761		1,151
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						35,959		26,886		51,594
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3						1,980,327				0
Ending Balance			1,771,532			20,388		1,917,319		1,771,532
Available-for-sale Securities [Member] | Investment Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					65,490	44,924		40,576		65,490
Purchases						0		0		0
Sales						(26,806)		0		(26,675)
Settlements						0		0		(205)
Realized and Unrealized Gains/(Losses) included in Income						6		(100)		(825)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						(394)		4,448		2,791
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3						0				0
Ending Balance			40,576			17,730		44,924		40,576
Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					5,091,600	4,152,605		3,659,281		5,091,600
Purchases						1,434,492		691,880		1,099,102
Sales						0		0		0
Settlements						(709,686)		(148,169)		(923,716)
Realized and Unrealized Gains/(Losses) included in Income						(20,944)		3,090		14,520
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						30,042		(53,477)		10,561
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3						32,824		0		1,632,786
Ending Balance			3,659,281			4,853,685		4,152,605		3,659,281
Available-for-sale Securities [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					1,553,669	1,888,344		1,731,222		1,553,669
Purchases						391,240		363,621		335,359
Sales						(97,954)		0		0
Settlements						(237,262)		(233,385)		(209,400)
Realized and Unrealized Gains/(Losses) included in Income						0		0		0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						35,959		26,886		51,594
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	[1]					1,980,327		0		0
Ending Balance			1,731,222			0		1,888,344		1,731,222
Trading Securities [Member] | Investment Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					928	491		689		928
Purchases						0		0		0
Sales						0		0		0
Settlements						(2,213)		(657)		(685)
Realized and Unrealized Gains/(Losses) included in Income						1,722		459		446
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3						0				0
Ending Balance			689			0		491		689
Trading Securities [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					58,344	28,975		40,310	[2]	58,344
Purchases						0		0		0
Sales						0		0		0
Settlements						(8,325)		(12,096)		(19,185)
Realized and Unrealized Gains/(Losses) included in Income						(262)	[2]	761	[3]	1,151	[4]
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3						0				0
Ending Balance			40,310	[2]		20,388	[2]	28,975		40,310	[2]
Institutional Credit [Member] | Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					5,070,366	4,121,244		3,631,662	[2]	5,070,366
Purchases						1,430,392		678,566		1,091,475
Sales						0		0		0
Settlements						(706,446)		(138,687)		(922,908)
Realized and Unrealized Gains/(Losses) included in Income						(20,944)		3,090		14,520
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						29,439		(53,387)		10,995
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3			0		1,600,000	0				1,632,786	[4]
Ending Balance			3,631,662	[2]		4,853,685	[2]	4,121,244		3,631,662	[2]
USDA Guarantees [Member] | Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					21,234	31,361		27,619		21,234
Purchases						4,100		13,314		7,627
Sales						0		0		0
Settlements						(3,240)		(9,482)		(808)
Realized and Unrealized Gains/(Losses) included in Income						0		0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						603		(90)		(434)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3						32,824	[1]			0
Ending Balance			27,619			0		31,361		27,619
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					65,285	44,924		40,576		65,285
Purchases						0		0		0
Sales						(26,806)		0		(26,675)
Settlements						0		0		0
Realized and Unrealized Gains/(Losses) included in Income						6		(100)		(825)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						(394)		4,448		2,791
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3						0				0
Ending Balance			40,576			17,730		44,924		40,576
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					205			0		205
Purchases										0
Sales										0
Settlements										(205)
Realized and Unrealized Gains/(Losses) included in Income										0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income										0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3										0
Ending Balance			0							0
Floating Interest Rate [Member] | Trading Securities [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance					$ 928	491		689	[5]	928
Purchases						0		0		0
Sales						0		0		0
Settlements						(2,213)		(657)	[6]	(685)
Realized and Unrealized Gains/(Losses) included in Income						1,722	[5]	459		446	[7]
Unrealized Gains/(Losses) included in Other Comprehen-sive Income						0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3						0				0
Ending Balance			$ 689	[5]		$ 0	[5]	$ 491		$ 689	[5]

[1] Includes $32.8 million of Farmer Mac Guaranteed USDA Securities and $2.0 billion of USDA Securities transferred from available-for-sale to held-to-maturity on October 1, 2016.

[2] Includes unrealized losses of $0.3 million attributable to assets still held as of December 31, 2016 that are recorded in "Gains on trading securities."

[3] Includes unrealized gains of $0.9 million attributable to assets still held as of December 31, 2015 that are recorded in "Gains on trading securities."

[4] Includes $1.6 billion of AgVantage securities transferred from available-for-sale to held-to-maturity on January 1, 2014 and $20.7 million of AgVantage securities purchased during 2014 transferred from available-for-sale to held-to-maturity.

[5] nrealized gains are attributable to assets still held as of December 31, 2016 and are recorded in "Gains on trading securities."

[6] Unrealized gains are attributable to assets still held as of December 31, 2015 and are recorded in "Gains on trading securities."

[7] Unrealized gains are attributable to assets still held as of December 31, 2014 and are recorded in "Gains on trading securities."