Fair Value Disclosures - Unobservable Input Reconciliation (Details) - USD ($)		3 Months Ended
	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 27,000			$ 0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [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
Cumulative Effect of New Accounting Principle in Period of Adoption	487,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		$ 5,508,576,000	4,891,803,000
Purchases		689,740,000	572,116,000
Sales		(34,293,000)	(32,589,000)
Settlements		(292,350,000)	(163,723,000)
Ending Balance	5,508,576,000	5,874,075,000	5,284,591,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(18,932,000)	(3,297,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		20,847,000	13,240,000
Fair Value, Inputs, Level 3 [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [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
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Trading Securities, Unrealized Gains		100,000	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		13,515,000	20,388,000
Purchases		34,293,000	32,589,000
Sales		(34,293,000)	(32,589,000)
Settlements		(1,973,000)	(1,704,000)
Ending Balance	13,515,000	11,558,000	18,602,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		16,000	(82,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0	0
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Investment Securities [Member] | Fair Value, Measurements, Recurring [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
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		23,147,000	17,730,000
Purchases		0	0
Sales		0	0
Settlements		(109,000)	(112,000)
Ending Balance	23,147,000	23,130,000	22,943,000
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)		92,000	(1,716,000)
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [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
Cumulative Effect of New Accounting Principle in Period of Adoption	487,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		5,471,914,000	4,853,685,000
Purchases		655,447,000	539,527,000
Sales		0	0
Settlements		(290,268,000)	(161,907,000)
Ending Balance	5,471,914,000	5,839,387,000	5,243,046,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(18,948,000)	(3,215,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		20,755,000	14,956,000
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3 [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [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
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0	0
Purchases		34,293,000	32,589,000
Sales		(34,293,000)	(32,589,000)
Settlements		0	0
Ending Balance	0	0	0
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)		0	0
Trading Securities [Member] | Fair Value, Inputs, Level 3 [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [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
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		13,515,000	20,388,000
Purchases		0	0
Sales		0	0
Settlements		(1,973,000)	(1,704,000)
Ending Balance	13,515,000	11,558,000	18,602,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		16,000	(82,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0	0
Institutional Credit [Member] | Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [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
Cumulative Effect of New Accounting Principle in Period of Adoption	487,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		5,471,914,000	4,853,685,000
Purchases		655,447,000	539,527,000
Sales		0	0
Settlements		(290,268,000)	(161,907,000)
Ending Balance	5,471,914,000	5,839,387,000	5,243,046,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(18,948,000)	(3,215,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		20,755,000	14,956,000
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3 [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 [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3			0
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		18,814,000	17,730,000
Purchases		0	0
Sales		0	0
Settlements		0	0
Ending Balance	18,814,000	19,010,000	18,124,000
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)		196,000	394,000
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [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
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		4,333,000	0
Purchases		0	0
Sales		0	0
Settlements		(109,000)	(112,000)
Ending Balance	$ 4,333,000	4,120,000	4,819,000
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)		$ (104,000)	$ (2,110,000)