Fair Value Disclosures - Quantitative Information (Details) - Fair Value, Measurements, Recurring [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 6,015,564	$ 5,322,025	$ 5,874,075	$ 5,508,576	$ 5,284,591	$ 4,891,803
USDA Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 10,748	$ 16,294	11,558	13,515	18,602	20,388
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	96.50%	95.50%
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	2.40%	2.10%
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	3.20%	3.60%
Fair Value Inputs, Prepayment Rate	7.00%	7.00%
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	96.50%	95.50%
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	3.80%	3.40%
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	5.30%	5.40%
Fair Value Inputs, Prepayment Rate	18.00%	19.00%
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	96.50%	95.50%
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	2.70%	2.40%
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	16.00%	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	$ 5,985,806	$ 5,282,562	5,839,387	5,471,914	5,243,046	4,853,685
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	0	0	0	0
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	5,985,806	5,282,562	5,839,387	5,471,914	5,243,046	4,853,685
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	$ 0	$ 4,651	4,120	4,333	4,819	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	3.10%	2.90%
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	3.10%	2.90%
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	3.10%	2.90%
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	$ 10,748	$ 16,294	$ 11,558	$ 13,515	$ 18,602	$ 20,388