Off-Balance Sheet Guarantees and Long Term Standby Purchase Commitments - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Guarantor Obligations [Line Items]
Loans and Leases Receivable, Impaired, Loans Past Due Period, Minimum	90 days
Term of Guarantees Maximum	30 years
Term of Guarantees Minimum	1 year
Maximum Potential Exposure on Guarantees including Long Term Standby Commitments	3,600,000,000	3,200,000,000
Commitments and Contingencies
Guarantee obligations issued prior to January 1, 2003 [Member]
Guarantor Obligations [Line Items]
Maximum Potential Exposure on Guarantees including Long Term Standby Commitments	474,700,000	188,000,000
Minimum [Member]
Guarantor Obligations [Line Items]
Failure to Repurchase USDA portion Period	30 days
Failure to Remit Borrower payment USDA portion Period	30 days
Alternate minimum [Member]
Guarantor Obligations [Line Items]
Loans and Leases Receivable, Impaired, Loans Past Due Period, Minimum	120 days
USDA Guaranteed Securities [Member] | Minimum [Member]
Guarantor Obligations [Line Items]
Impaired Finance Receivables, Period Past Due for Acquired Delinquent Loans	60 days
Farmer Mac I & II [Member]
Guarantor Obligations [Line Items]
Commitments and Contingencies	71,400,000	48,400,000
Long-Term Standby Purchase Commitments [Member]
Guarantor Obligations [Line Items]
Loans and Leases Receivable, Impaired, Loans Past Due Period, Minimum	90 days
Long-Term Standby Purchase Commitments [Member] | Alternate minimum [Member]
Guarantor Obligations [Line Items]
Impaired Finance Receivables, Period Past Due for Acquired Delinquent Loans	120 days
Rural Utilities [Member]
Guarantor Obligations [Line Items]
Commitments and Contingencies	5,300,000	17,700,000
Long-Term Standby Purchase Commitments On Or After January 1, 2003 [Member] | Collateralized Loan Obligations [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	22,000,000	15,100,000
Long-Term Standby Purchase Commitments [Member] | Collateralized Loan Obligations [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	2,200,000,000	1,800,000,000
Variable Interest Entity, Not Primary Beneficiary | Long-Term Standby Purchase Commitments [Member] | Collateralized Loan Obligations [Member]
Guarantor Obligations [Line Items]
Securitization Interests Continued To Be Held By Transferor, Weighted Average Remaining Maturity	13 years 7 months 6 days	13 years 9 months 18 days