Allowance for Losses and Concentration of Credit Risk - Allowance for losses activity (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Eligible Loans [Member]	Sep. 30, 2011 Eligible Loans [Member]	Sep. 30, 2012 Eligible Loans [Member]	Sep. 30, 2011 Eligible Loans [Member]	Sep. 30, 2012 Long-Term Standby Purchase Commitments and Farmer Mac Guaranteed Securities	Sep. 30, 2011 Long-Term Standby Purchase Commitments and Farmer Mac Guaranteed Securities	Sep. 30, 2012 Long-Term Standby Purchase Commitments and Farmer Mac Guaranteed Securities	Sep. 30, 2011 Long-Term Standby Purchase Commitments and Farmer Mac Guaranteed Securities	Sep. 30, 2012 Crops	Sep. 30, 2011 Crops	Sep. 30, 2012 Crops	Sep. 30, 2011 Crops	Sep. 30, 2012 Permanent Plantings	Sep. 30, 2011 Permanent Plantings	Sep. 30, 2012 Permanent Plantings	Sep. 30, 2011 Permanent Plantings	Sep. 30, 2012 Livestock	Sep. 30, 2011 Livestock	Sep. 30, 2012 Livestock	Sep. 30, 2011 Livestock	Sep. 30, 2012 Part-Time Farm	Sep. 30, 2011 Part-Time Farm	Sep. 30, 2012 Part-Time Farm	Sep. 30, 2011 Part-Time Farm	Sep. 30, 2012 Ag Storage and Processing (including ethanol facilities)	Sep. 30, 2011 Ag Storage and Processing (including ethanol facilities)	Sep. 30, 2012 Ag Storage and Processing (including ethanol facilities)	Sep. 30, 2011 Ag Storage and Processing (including ethanol facilities)	Sep. 30, 2012 Other	Sep. 30, 2011 Other	Sep. 30, 2012 Other	Sep. 30, 2011 Other	Sep. 30, 2012 Specific reserve reclassification [Member] Eligible Loans [Member]	Dec. 31, 2010 Specific reserve reclassification [Member] LTSPCs
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance, Allowance for Loan Losses			$ 10,161,000		$ 9,361,000	$ 11,053,000	$ 10,161,000	$ 9,803,000
Beginning Balance, Reserve for Losses			7,355,000						8,779,000	7,443,000	7,355,000	10,312,000
Beginning Balance, Total Allowance	18,140,000	18,496,000	17,516,000	20,115,000									4,281,000	3,715,000	4,133,000	3,572,000	2,522,000	3,582,000	3,099,000	3,537,000	1,471,000	2,688,000	1,697,000	2,749,000	557,000	402,000	477,000	445,000	9,302,000	8,100,000	8,106,000	9,797,000	7,000	9,000	4,000	15,000
(Relase of)/provision for losses, Allowance for Loan Losses	137,000	(349,000)	(663,000)	1,092,000	137,000		(663,000)			(349,000)		1,092,000
(Release of)/provision for losses, Reserve for Losses	(43,000)	(452,000)	1,381,000	(3,321,000)					(43,000)	(452,000)	1,381,000	(3,321,000)
(Release of)/provision for losses, Total Allowance	94,000	(801,000)	718,000	(2,229,000)									(305,000)	144,000	(157,000)	463,000	176,000	(27,000)	(401,000)	25,000	(129,000)	(891,000)	(355,000)	(944,000)	192,000	(28,000)	272,000	(71,000)	161,000	2,000	1,357,000	(1,695,000)	(1,000)	(1,000)	2,000	(7,000)	(300,000)	1,800,000
Charge-offs, Allowance for Losses					(448,000)	(5,000)	(448,000)	(196,000)
Charge-offs, Reserve for Losses									0	0	0	0
Charge-offs, Total Allowance	(448,000)	(5,000)	(448,000)	(196,000)									0	0	0	(176,000)	(375,000)	0	(375,000)	(7,000)	0	0	0	(8,000)	(73,000)	(5,000)	(73,000)	(5,000)	0	0	0	0	0	0	0	0
Ending Balance, Allowance for Loan Losses	9,050,000		9,050,000		9,050,000	10,699,000	9,050,000	10,699,000
Ending Balance, Reserve for Losses	8,736,000		8,736,000						8,736,000	6,991,000	8,736,000	6,991,000
Ending Balance, Total Allowance	$ 17,786,000	$ 17,690,000	$ 17,786,000	$ 17,690,000									$ 3,976,000	$ 3,859,000	$ 3,976,000	$ 3,859,000	$ 2,323,000	$ 3,555,000	$ 2,323,000	$ 3,555,000	$ 1,342,000	$ 1,797,000	$ 1,342,000	$ 1,797,000	$ 676,000	$ 369,000	$ 676,000	$ 369,000	$ 9,463,000	$ 8,102,000	$ 9,463,000	$ 8,102,000	$ 6,000	$ 8,000	$ 6,000	$ 8,000