Fair Value Disclosures - Quantitative Information (Details) - Recurring - Level 3 $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 6,973,567	$ 7,170,850	$ 6,003,211	$ 5,508,576
Floating rate auction-rate certificates backed by Government guaranteed student loans | Floating Interest Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	19,171	18,912
USDA Securities:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	6,695	8,913	$ 9,999	$ 13,515
AgVantage Securities | Farmer Mac Guaranteed Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 6,947,701	$ 7,143,025
Range of broker quotes | Minimum | Floating rate auction-rate certificates backed by Government guaranteed student loans | Valuation, Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.975	0.960
Range of broker quotes | Maximum | Floating rate auction-rate certificates backed by Government guaranteed student loans | Valuation, Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.975	0.960
Range of broker quotes | Weighted Average | Floating rate auction-rate certificates backed by Government guaranteed student loans | Valuation, Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.975	0.960
Discount rate | Minimum | AgVantage Securities | Farmer Mac Guaranteed Securities | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.008	0.023
Discount rate | Minimum | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.009	0.023
Discount rate | Maximum | AgVantage Securities | Farmer Mac Guaranteed Securities | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.023	0.055
Discount rate | Maximum | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.019	0.026
Discount rate | Weighted Average | AgVantage Securities | Farmer Mac Guaranteed Securities | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.013	0.026
Discount rate | Weighted Average | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.014	0.021
CPR | Minimum | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.25	0.10
CPR | Maximum | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.49	0.21
CPR | Weighted Average | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.44	0.19