Fair Value Disclosures - Quantitative Information (Details) - Fair Value, Measurements, Recurring [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 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	$ 5,986,502	$ 5,508,576	$ 6,015,564	$ 5,481,586	$ 5,322,025	$ 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	$ 10,237	$ 13,515	10,748	14,864	16,294	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	96.30%	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	2.90%	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	3.30%	3.60%
Fair Value Inputs, Prepayment Rate	6.00%	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	96.30%	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	4.00%	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	5.20%	5.40%
Fair Value Inputs, Prepayment Rate	16.00%	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	96.30%	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	3.10%	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	4.90%	5.00%
Fair Value Inputs, Prepayment Rate	15.00%	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	$ 5,957,304	$ 5,471,914	5,985,806	5,443,746	5,282,562	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	5,957,304	5,471,914	5,985,806	5,443,746	5,282,562	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	$ 4,333		4,458	4,651	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	0.00%	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	0.00%	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	0.00%	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	$ 10,237	$ 13,515	$ 10,748	$ 14,864	$ 16,294	$ 20,388