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, 2017		Dec. 31, 2016		Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 5,481,586		$ 4,891,803		$ 5,322,025	$ 6,958,879	$ 6,741,588	$ 6,115,339
USDA Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 14,864		$ 20,388		16,294	2,003,816	1,992,546	1,917,319
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	94.00%		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	2.00%		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	94.00%		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	3.30%		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	5.30%		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	94.00%		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	2.20%		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	$ 5,443,746		$ 4,853,685		5,282,562	4,937,481	4,731,031	4,152,605
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	1,980,327	1,967,759	1,888,344
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	5,443,746	[1]	4,853,685	[1]	5,282,562	4,904,657	4,697,584	4,121,244
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						32,824	33,447	31,361
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	$ 4,458		$ 0		4,651
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	3.70%		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	3.70%		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	3.70%		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	$ 14,864	[1]	$ 20,388	[1]	$ 16,294	$ 23,489	$ 24,787	$ 28,975

[1] Includes unrealized gains of $34,000 attributable to assets still held as of September 30, 2017 that are recorded in "Gains/(losses) on trading securities."