Fair Value Disclosures - Quantitative Information (Details) - Fair Value, Measurements, Recurring - Level 3 $ in Thousands	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 7,924,456	$ 7,612,315	$ 7,170,850	$ 7,064,077	$ 6,470,023	$ 6,003,211
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	18,283		18,912
USDA Securities:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	7,786		8,913
Institutional Credit | Farmer Mac Guaranteed Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	7,898,387		7,143,025
USDA Guarantees | USDA Securities:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 7,786	$ 8,408	$ 8,913	$ 9,201	$ 9,487	$ 9,999
Measurement Input, 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	0.930		0.960
Measurement Input, 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	0.930		0.960
Measurement Input, 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	0.930		0.960
Measurement Input, Discount Rate | Minimum | Institutional Credit | Farmer Mac Guaranteed Securities | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.009		0.023
Measurement Input, Discount Rate | Minimum | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.015		0.023
Measurement Input, Discount Rate | Maximum | Institutional Credit | Farmer Mac Guaranteed Securities | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.040		0.055
Measurement Input, Discount Rate | Maximum | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.022		0.026
Measurement Input, Discount Rate | Weighted Average | Institutional Credit | Farmer Mac Guaranteed Securities | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.012		0.026
Measurement Input, Discount Rate | Weighted Average | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.016		0.021
Measurement Input, Constant Prepayment Rate | Minimum | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.14		0.10
Measurement Input, Constant Prepayment Rate | Maximum | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.23		0.21
Measurement Input, Constant Prepayment Rate | Weighted Average | USDA Guarantees | USDA Securities: | Valuation, Cost Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.22		0.19