Fair Value Disclosures - Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 [Member] - Fair Value, Measurements, Recurring [Member] - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	$ 5,874,075,000	$ 5,284,591,000	$ 5,508,576,000	4,891,803,000
Purchases	348,531,000	247,794,000	1,038,271,000	819,910,000
Sales	(45,014,000)	(53,506,000)	(79,307,000)	(86,095,000)
Settlements	(152,042,000)	(167,665,000)	(444,392,000)	(331,388,000)
Ending Balance	6,015,564,000	5,322,025,000	6,015,564,000	5,322,025,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(12,931,000)	9,550,000	(31,863,000)	6,253,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	2,945,000	1,261,000	23,792,000	14,501,000
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
Trading Securities, Unrealized Gains	11,000	100,000	100,000	7,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	11,558,000	18,602,000	13,515,000	20,388,000
Purchases	45,014,000	53,506,000	79,307,000	86,095,000
Sales	(45,014,000)	(53,506,000)	(79,307,000)	(86,095,000)
Settlements	(821,000)	(2,306,000)	(2,794,000)	(4,010,000)
Ending Balance	10,748,000	16,294,000	10,748,000	16,294,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	11,000	(2,000)	27,000	(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	23,130,000	22,943,000	23,147,000	17,730,000
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	(2,028,000)	(111,000)	(2,137,000)	(223,000)
Ending Balance	19,010,000	23,169,000	19,010,000	23,169,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(2,092,000)	0	(2,092,000)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	337,000	92,000	(1,379,000)
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	5,839,387,000	5,243,046,000	5,471,914,000	4,853,685,000
Purchases	303,517,000	194,288,000	958,964,000	733,815,000
Sales	0	0	0	0
Settlements	(149,193,000)	(165,248,000)	(439,461,000)	(327,155,000)
Ending Balance	5,985,806,000	5,282,562,000	5,985,806,000	5,282,562,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(10,850,000)	9,552,000	(29,798,000)	6,337,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	2,945,000	924,000	23,700,000	15,880,000
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	45,014,000	53,506,000	79,307,000	86,095,000
Sales	(45,014,000)	(53,506,000)	(79,307,000)	(86,095,000)
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	11,558,000	18,602,000	13,515,000	20,388,000
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	(821,000)	(2,306,000)	(2,794,000)	(4,010,000)
Ending Balance	10,748,000	16,294,000	10,748,000	16,294,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	11,000	(2,000)	27,000	(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	5,839,387,000	5,243,046,000	5,471,914,000	4,853,685,000
Purchases	303,517,000	194,288,000	958,964,000	733,815,000
Sales	0	0	0	0
Settlements	(149,193,000)	(165,248,000)	(439,461,000)	(327,155,000)
Ending Balance	5,985,806,000	5,282,562,000	5,985,806,000	5,282,562,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(10,850,000)	9,552,000	(29,798,000)	6,337,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	2,945,000	924,000	23,700,000	15,880,000
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	18,124,000	18,814,000	17,730,000
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Ending Balance	19,010,000	18,518,000	19,010,000	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)	0	394,000	196,000	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	4,120,000	4,819,000	4,333,000	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	(2,028,000)	(111,000)	(2,137,000)	(223,000)
Ending Balance	0	4,651,000	0	4,651,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(2,092,000)	0	(2,092,000)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	$ 0	$ (57,000)	$ (104,000)	$ (2,167,000)