Loans and Allowance for Losses and Concentrations of Credit Risk - Certain Loans Acquired Delinquencies and Net Credit Losses (Details) (USD $)	9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired Financing Receivable, Unpaid Principal Balance	$ 85,143,000			$ 98,837,000
Net credit losses (recoveries)	2,825,000		199,000
Loans and Leases Receivable, Impaired, Loans Past Due Period, Minimum	90 days
90-day delinquencies [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired Financing Receivable, Unpaid Principal Balance	33,042,000	[1]		33,263,000	[1]
90-day delinquencies [Member] | Eligible Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities, Subject To Be Purchased	1,400,000	[1]		4,600,000	[1]
On-balance sheet [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net credit losses (recoveries)	2,825,000		199,000
On-balance sheet [Member] | Eligible Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net credit losses (recoveries)	2,825,000		199,000
On-balance sheet [Member] | 90-day delinquencies [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired Financing Receivable, Unpaid Principal Balance	32,711,000	[1]		29,592,000	[1]
On-balance sheet [Member] | 90-day delinquencies [Member] | Eligible Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired Financing Receivable, Unpaid Principal Balance	32,711,000	[1]		29,592,000	[1]
Off-balance sheet [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net credit losses (recoveries)	0		0
Off-balance sheet [Member] | Long-Term Standby Purchase Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net credit losses (recoveries)	0		0
Off-balance sheet [Member] | 90-day delinquencies [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired Financing Receivable, Unpaid Principal Balance	331,000	[1]		3,671,000	[1]
Off-balance sheet [Member] | 90-day delinquencies [Member] | Long-Term Standby Purchase Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired Financing Receivable, Unpaid Principal Balance	$ 331,000	[1]		$ 3,671,000	[1]

[1] Includes loans and loans underlying Farm & Ranch Farmer Mac Guaranteed Securities and LTSPCs that are 90 days or more past due, in foreclosure, restructured after delinquency, and in bankruptcy, excluding loans performing under either their original loan terms or a court-approved bankruptcy plan.