Fair Value Disclosures - Quantitative Information (Details) - Fair Value, Measurements, Recurring [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 5,508,576	$ 4,891,803	$ 6,115,339	$ 5,472,078
USDA Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 13,515	$ 20,388	1,917,319	1,771,532
Minimum [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Market Approach Valuation Technique [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Range of Offered Quotes	95.50%	90.00%
Minimum [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member] | Cost Approach Valuation Technique [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	2.10%	1.50%
Minimum [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Cost Approach Valuation Technique [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	3.60%	4.00%
Fair Value Inputs, Prepayment Rate	7.00%	13.00%
Maximum [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Market Approach Valuation Technique [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Range of Offered Quotes	95.50%	90.00%
Maximum [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member] | Cost Approach Valuation Technique [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	3.40%	3.30%
Maximum [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Cost Approach Valuation Technique [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	5.40%	5.30%
Fair Value Inputs, Prepayment Rate	19.00%	18.00%
Weighted Average [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Market Approach Valuation Technique [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Range of Offered Quotes	95.50%	90.00%
Weighted Average [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member] | Cost Approach Valuation Technique [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	2.40%	1.90%
Weighted Average [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Cost Approach Valuation Technique [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	5.00%	5.00%
Fair Value Inputs, Prepayment Rate	17.00%	17.00%
Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 5,471,914	$ 4,853,685	4,152,605	3,659,281
Available-for-sale Securities [Member] | USDA Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0	0	1,888,344	1,731,222
Available-for-sale Securities [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	5,471,914	4,853,685	4,121,244	3,631,662
Available-for-sale Securities [Member] | USDA Guarantees [Member] | Farmer Mac Guaranteed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0	31,361	27,619
Available-for-sale Securities [Member] | Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 4,333	$ 0
Fair Value Inputs, Prepayment Rate	0.00%	0.00%
Available-for-sale Securities [Member] | Minimum [Member] | Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	2.90%	0.00%
Available-for-sale Securities [Member] | Maximum [Member] | Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	2.90%	0.00%
Available-for-sale Securities [Member] | Weighted Average [Member] | Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	2.90%	0.00%
Trading Securities [Member] | USDA Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 13,515	$ 20,388	$ 28,975	$ 40,310