Fair Value Disclosures - Narrative (Details) (USD $)	3 Months Ended							12 Months Ended			15 Months Ended			0 Months Ended		12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Dec. 31, 2012 Level 3	Dec. 31, 2011 Level 3	Dec. 31, 2012 Asset-backed Securities [Member] Floating Interest Rate [Member]	Dec. 31, 2011 Asset-backed Securities [Member] Floating Interest Rate [Member]	Dec. 31, 2010 Fair Value, Measurements, Recurring [Member] GSE Subordinated Debt Floating Interest Rate [Member] Level 3	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 3	Dec. 31, 2010 Fair Value, Measurements, Recurring [Member] Level 3	Dec. 31, 2010 CoBank, ACB [Member] Fair Value, Measurements, Recurring [Member] Preferred Stock [Member] Fixed Interest Rate [Member] Level 3	Dec. 31, 2010 AgFirst Farm Credit Bank [Member] Fair Value, Measurements, Recurring [Member] Preferred Stock [Member] Fixed Interest Rate [Member] Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Reset period of auction rate certificate interest rates								28 days
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3																$ 70,000,000			$ 88,500,000	$ 88,000,000
Derivative asset and liability credit risk valuation adjustment	600,000				200,000			600,000	200,000
Lower of cost or fair value adjustment on loans held for sale	(5,943,000)	0	0	0	0	(156,000)	(808,000)	(5,943,000)	8,887,000	(8,748,000)	9,851,000
Assets and Liabilities, Fair Value Disclosure												7,100,000,000	5,900,000,000
Assets and liabilities fair value as percent of total assets												56.00%	49.00%
Assets and liabilities fair value as percent of total investments												73.00%	72.00%
Trading Securities, Unrealized Holding Loss														3,080,000	3,342,000		1,800,000	2,000,000
Changes in fair value gain (losses), fair value option								$ 45,000,000	$ (2,200,000)	$ (4,900,000)