Fair Value Disclosures - Quantitative Information (Details) - Fair Value, Measurements, Recurring [Member] - Fair Value, Inputs, Level 3 [Member] $ in Thousands	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 7,210,692	$ 7,064,077	$ 6,003,211	$ 5,986,502	$ 6,015,564	$ 5,508,576
USDA Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	8,943	9,201	9,999	10,237	10,748	13,515
Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	7,182,541	7,035,668	5,974,497	5,957,304	5,985,806	5,471,914
Available-for-sale Securities [Member] | USDA Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	0	0	0	0	0	0
Available-for-sale Securities [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	7,182,541	7,035,668	5,974,497	5,957,304	5,985,806	5,471,914
Trading Securities [Member] | USDA Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 8,943	$ 9,201	$ 9,999	$ 10,237	$ 10,748	$ 13,515
Measurement Input, Range of Broker Quotes [Member] | Minimum [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.975		0.950
Measurement Input, Range of Broker Quotes [Member] | Maximum [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.975		0.950
Measurement Input, Range of Broker Quotes [Member] | Weighted Average [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.975		0.950
Measurement Input, Discount Rate [Member] | Minimum [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.022		0.030
Measurement Input, Discount Rate [Member] | Minimum [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.024		0.032
Measurement Input, Discount Rate [Member] | Maximum [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.038		0.044
Measurement Input, Discount Rate [Member] | Maximum [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.033		0.052
Measurement Input, Discount Rate [Member] | Weighted Average [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.026		0.033
Measurement Input, Discount Rate [Member] | Weighted Average [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.033		0.049
Measurement Input, Constant Prepayment Rate [Member] | Minimum [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.11		0.07
Measurement Input, Constant Prepayment Rate [Member] | Maximum [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.21		0.17
Measurement Input, Constant Prepayment Rate [Member] | Weighted Average [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.20		0.16