Allowance for Losses and Concentration of Credit Risk - Impairment Method (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Jun. 30, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Financing Receivable, Allowance for Credit Losses [Line Items]
Evaluated collectively for impairment	$ 4,294,453			$ 4,261,137
Evaluated individually for impairment	108,504			88,026
Total financing receivable	4,402,957	[1]		4,349,163	[1]
Allowance for Losses, evaluated collectively for impairment	9,775			10,220
Allowance for Losses, evaluated individually for impairment	8,011			7,296
Total allowance for losses	17,786		18,140	17,516		17,690	18,496	20,115
Crops
Financing Receivable, Allowance for Credit Losses [Line Items]
Evaluated collectively for impairment	1,925,509			1,835,439
Evaluated individually for impairment	31,786			29,520
Total financing receivable	1,957,295	[1]		1,864,959	[1]
Allowance for Losses, evaluated collectively for impairment	1,626			1,723
Allowance for Losses, evaluated individually for impairment	2,350			2,410
Total allowance for losses	3,976		4,281	4,133		3,859	3,715	3,572
Permanent Plantings
Financing Receivable, Allowance for Credit Losses [Line Items]
Evaluated collectively for impairment	795,516			796,100
Evaluated individually for impairment	37,939			28,245
Total financing receivable	833,455	[1]		824,345	[1]
Allowance for Losses, evaluated collectively for impairment	643			775
Allowance for Losses, evaluated individually for impairment	1,680			2,324
Total allowance for losses	2,323		2,522	3,099		3,555	3,582	3,537
Livestock
Financing Receivable, Allowance for Credit Losses [Line Items]
Evaluated collectively for impairment	1,169,443			1,213,227
Evaluated individually for impairment	17,034			10,884
Total financing receivable	1,186,477	[1]		1,224,111	[1]
Allowance for Losses, evaluated collectively for impairment	1,131			1,290
Allowance for Losses, evaluated individually for impairment	211			407
Total allowance for losses	1,342		1,471	1,697		1,797	2,688	2,749
Part-Time Farm
Financing Receivable, Allowance for Credit Losses [Line Items]
Evaluated collectively for impairment	198,175			232,607
Evaluated individually for impairment	16,391			12,513
Total financing receivable	214,566	[1]		245,120	[1]
Allowance for Losses, evaluated collectively for impairment	107			172
Allowance for Losses, evaluated individually for impairment	569			305
Total allowance for losses	676		557	477		369	402	445
Ag Storage and Processing (including ethanol facilities)
Financing Receivable, Allowance for Credit Losses [Line Items]
Evaluated collectively for impairment	193,769			167,850
Evaluated individually for impairment	4,337			5,842
Total financing receivable	198,106	[1]		173,692	[1]
Allowance for Losses, evaluated collectively for impairment	6,263			6,256
Allowance for Losses, evaluated individually for impairment	3,200			1,850
Total allowance for losses	9,463		9,302	8,106		8,102	8,100	9,797
Other
Financing Receivable, Allowance for Credit Losses [Line Items]
Evaluated collectively for impairment	12,041			15,914
Evaluated individually for impairment	1,017			1,022
Total financing receivable	13,058	[1]		16,936	[1]
Allowance for Losses, evaluated collectively for impairment	5			4
Allowance for Losses, evaluated individually for impairment	1			0
Total allowance for losses	$ 6		$ 7	$ 4		$ 8	$ 9	$ 15

[1] Amounts represent unpaid principal balance of risk-rated loans, which is the basis Farmer Mac uses to analyze its portfolio, and recorded investment of past due loans.