Fair Value Disclosures - Quantitative Information (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Derivative Liability	$ 40,207	$ 64,388
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Derivative Liability	0	235
Market Approach Valuation Technique [Member] | Minimum [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Range of Offered Quotes	82.00%	82.00%
Market Approach Valuation Technique [Member] | Maximum [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Range of Offered Quotes	92.00%	92.00%
Market Approach Valuation Technique [Member] | Weighted Average [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Range of Offered Quotes	87.70%	88.10%
Market Approach Valuation Technique [Member] | Floating Interest Rate [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale, at fair value	40,774	65,285
Cost Approach Valuation Technique [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Derivative Liability		235
Cost Approach Valuation Technique [Member] | Minimum [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	12.20%	13.00%
Cost Approach Valuation Technique [Member] | Minimum [Member] | Collateralized Mortgage Obligations [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	1.70%	1.80%
Cost Approach Valuation Technique [Member] | Maximum [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	20.90%	22.50%
Cost Approach Valuation Technique [Member] | Maximum [Member] | Collateralized Mortgage Obligations [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	1.70%	1.80%
Cost Approach Valuation Technique [Member] | Weighted Average [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	16.60%	17.70%
Cost Approach Valuation Technique [Member] | Weighted Average [Member] | Collateralized Mortgage Obligations [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	1.70%	1.80%
Cost Approach Valuation Technique [Member] | Floating Interest Rate [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading, at fair value	777	928
Cost Approach Valuation Technique [Member] | Floating Interest Rate [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Prepayment Rate	10.00%	10.00%
Cost Approach Valuation Technique [Member] | Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale, at fair value	187	205
Cost Approach Valuation Technique [Member] | Floating Interest Rate [Member] | Collateralized Mortgage Obligations [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Prepayment Rate	6.00%	6.00%
Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale, at fair value	3,336,326	5,070,366
Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | Minimum [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	0.80%	0.90%
Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | Minimum [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	0.80%	0.90%
Fair Value Inputs, Prepayment Rate	8.00%	7.00%
Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | Maximum [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	2.20%	3.60%
Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | Maximum [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	3.20%	3.20%
Fair Value Inputs, Prepayment Rate	15.00%	14.00%
Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | Weighted Average [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	1.50%	1.80%
Institutional Credit [Member] | Cost Approach Valuation Technique [Member] | Weighted Average [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	1.80%	1.90%
Fair Value Inputs, Prepayment Rate	6.00%	11.00%
USDA Guarantees [Member] | Cost Approach Valuation Technique [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale, at fair value	20,449	21,234
USDA Guarantees [Member] | Cost Approach Valuation Technique [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Marketable Securities	$ 1,730,845	$ 1,612,013
USDA Guarantees [Member] | Cost Approach Valuation Technique [Member] | Minimum [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	1.10%	1.20%
Fair Value Inputs, Prepayment Rate	0.00%	0.00%
USDA Guarantees [Member] | Cost Approach Valuation Technique [Member] | Maximum [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	5.20%	5.30%
Fair Value Inputs, Prepayment Rate	15.00%	23.00%
USDA Guarantees [Member] | Cost Approach Valuation Technique [Member] | Weighted Average [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	3.20%	3.40%
Fair Value Inputs, Prepayment Rate	6.00%	5.00%
Derivative Financial Instruments, Liabilities [Member] | Cost Approach Valuation Technique [Member] | Minimum [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	0.60%	0.70%
Fair Value Inputs, Prepayment Rate	10.00%	10.00%
Derivative Financial Instruments, Liabilities [Member] | Cost Approach Valuation Technique [Member] | Maximum [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	2.40%	2.30%
Fair Value Inputs, Prepayment Rate	11.00%	11.00%
Derivative Financial Instruments, Liabilities [Member] | Cost Approach Valuation Technique [Member] | Weighted Average [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate	1.20%	1.30%
Fair Value Inputs, Prepayment Rate	10.00%	10.00%