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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				$ 7,041,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 6,015,564,000	$ 5,322,025,000	$ 5,508,576,000	4,891,803,000
Purchases	713,702,000	234,644,000	1,751,973,000	1,054,554,000
Sales	(26,321,000)	(40,844,000)	(105,628,000)	(126,939,000)
Settlements	(689,721,000)	(31,392,000)	(1,134,113,000)	(362,780,000)
Ending Balance	5,986,502,000	5,481,586,000	5,986,502,000	5,481,586,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(13,435,000)	(1,171,000)	(45,298,000)	5,082,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(13,287,000)	(1,676,000)	10,505,000	12,825,000
USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading Securities, Unrealized Holding Loss	(9,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				0
Trading Securities, Unrealized Gains		34,000	100,000	42,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	10,748,000	16,294,000	13,515,000	20,388,000
Purchases	26,321,000	40,844,000	105,628,000	126,939,000
Sales	(26,321,000)	(40,844,000)	(105,628,000)	(126,939,000)
Settlements	(508,000)	(1,430,000)	(3,302,000)	(5,440,000)
Ending Balance	10,237,000	14,864,000	10,237,000	14,864,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(3,000)	0	24,000	(84,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0	0	0
Available-for-sale Securities [Member] | Investment Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				7,041,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	19,010,000	23,169,000	23,147,000	17,730,000
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	(111,000)	(2,137,000)	(334,000)
Ending Balance	18,961,000	22,976,000	18,961,000	22,976,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	0	(2,092,000)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(49,000)	(82,000)	43,000	(1,461,000)
Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5,985,806,000	5,282,562,000	5,471,914,000	4,853,685,000
Purchases	687,381,000	193,800,000	1,646,345,000	927,615,000
Sales	0	0	0	0
Settlements	(689,213,000)	(29,851,000)	(1,128,674,000)	(357,006,000)
Ending Balance	5,957,304,000	5,443,746,000	5,957,304,000	5,443,746,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(13,432,000)	(1,171,000)	(43,230,000)	5,166,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(13,238,000)	(1,594,000)	10,462,000	14,286,000
Available-for-sale Securities [Member] | USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0	0	0	0
Purchases	26,321,000	40,844,000	105,628,000	126,939,000
Sales	(26,321,000)	(40,844,000)	(105,628,000)	(126,939,000)
Settlements	0	0	0	0
Ending Balance	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0		0
Trading Securities [Member] | USDA Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	10,748,000	16,294,000	13,515,000	20,388,000
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	(508,000)	(1,430,000)	(3,302,000)	(5,440,000)
Ending Balance	10,237,000	14,864,000	10,237,000	14,864,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(3,000)	0	24,000	(84,000)
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 [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5,985,806,000	5,282,562,000	5,471,914,000	4,853,685,000
Purchases	687,381,000	193,800,000	1,646,345,000	927,615,000
Sales	0	0	0	0
Settlements	(689,213,000)	(29,851,000)	(1,128,674,000)	(357,006,000)
Ending Balance	5,957,304,000	5,443,746,000	5,957,304,000	5,443,746,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(13,432,000)	(1,171,000)	(43,230,000)	5,166,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(13,238,000)	(1,594,000)	10,462,000	14,286,000
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 [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	19,010,000	18,518,000	18,814,000	17,730,000
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Ending Balance	18,961,000	18,518,000	18,961,000	18,518,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(49,000)	0	147,000	788,000
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 [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				7,041,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		4,651,000	4,333,000	0
Purchases		0	0	0
Sales		0	0	0
Settlements		(111,000)	(2,137,000)	(334,000)
Ending Balance	$ 0	4,458,000	0	4,458,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0	(2,092,000)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		$ (82,000)	$ (104,000)	$ (2,249,000)