Accounting Policies - Variable Interest Entities (Details) (Variable Interest Entity, Not Primary Beneficiary, USD $)	Sep. 30, 2012	Dec. 31, 2011
Variable Interest Entity [Line Items]
Variable interest entity non-consolidated off balance sheet guaranteed securities	$ 2,000,000,000	$ 1,600,000,000
Farmer Mac Guaranteed Securities Trusts
Variable Interest Entity [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	61,400,000	66,600,000
Variable interest entity maximum loss exposure	59,000,000	65,400,000
Investment Securities Trusts
Variable Interest Entity [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	800,000,000	800,000,000
Variable interest entity maximum loss exposure	$ 800,000,000	$ 800,000,000