Allowance for Losses and Concentration of Credit Risk - Impaired Financing Receivables (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	$ 21,888		$ 21,888		$ 24,295
with specific allowance, Recorded investment	78,802		78,802		64,068
Total Recorded investment	100,690		100,690		88,363
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	21,702		21,702		25,691
With specific allowance, Unpaid principal balance	77,053		77,053		62,335
Total Unpaid principal balance	98,755		98,755		88,026
Associated Allowance	8,116		8,116		7,296
Recorded Investment of Loans on Nonaccrual Status	44,851		44,851		45,338
Average recorded investment in impaired loans	102,033	89,133	97,475	94,431
Income recognized on impaired loans	586	293	1,107	1,116
Crops
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	3,142		3,142		6,809
with specific allowance, Recorded investment	27,070		27,070		23,009
Total Recorded investment	30,212		30,212		29,818
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	3,110		3,110		7,446
With specific allowance, Unpaid principal balance	25,892		25,892		22,074
Total Unpaid principal balance	29,002		29,002		29,520
Associated Allowance	2,619		2,619		2,410
Recorded Investment of Loans on Nonaccrual Status	8,824		8,824		9,214
Average recorded investment in impaired loans	27,873	30,040	28,521	32,016
Income recognized on impaired loans	64	59	141	215
Permanent Plantings
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	9,073		9,073		10,083
with specific allowance, Recorded investment	21,696		21,696		18,668
Total Recorded investment	30,769		30,769		28,751
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	9,041		9,041		9,957
With specific allowance, Unpaid principal balance	21,516		21,516		18,288
Total Unpaid principal balance	30,557		30,557		28,245
Associated Allowance	1,467		1,467		2,075
Recorded Investment of Loans on Nonaccrual Status	23,566		23,566		25,710
Average recorded investment in impaired loans	35,012	29,531	32,925	30,188
Income recognized on impaired loans	376	186	676	213
Livestock
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	5,853		5,853		3,248
with specific allowance, Recorded investment	11,897		11,897		7,036
Total Recorded investment	17,750		17,750		10,284
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	5,756		5,756		4,088
With specific allowance, Unpaid principal balance	11,709		11,709		6,796
Total Unpaid principal balance	17,465		17,465		10,884
Associated Allowance	341		341		513
Recorded Investment of Loans on Nonaccrual Status	4,397		4,397		3,483
Average recorded investment in impaired loans	16,036	12,990	14,118	14,597
Income recognized on impaired loans	63	29	116	246
Part-Time Farm
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	2,895		2,895		3,241
with specific allowance, Recorded investment	13,623		13,623		9,392
Total Recorded investment	16,518		16,518		12,633
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	2,894		2,894		3,298
With specific allowance, Unpaid principal balance	13,483		13,483		9,215
Total Unpaid principal balance	16,377		16,377		12,513
Associated Allowance	638		638		448
Recorded Investment of Loans on Nonaccrual Status	8,064		8,064		6,931
Average recorded investment in impaired loans	17,671	9,391	15,992	10,235
Income recognized on impaired loans	83	19	174	60
Ag Storage and Processing (including ethanol facilities)
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	0		0		0
with specific allowance, Recorded investment	4,399		4,399		5,842
Total Recorded investment	4,399		4,399		5,842
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	0		0		0
With specific allowance, Unpaid principal balance	4,337		4,337		5,842
Total Unpaid principal balance	4,337		4,337		5,842
Associated Allowance	3,050		3,050		1,850
Recorded Investment of Loans on Nonaccrual Status	0		0		0
Average recorded investment in impaired loans	4,400	6,458	4,880	6,578
Income recognized on impaired loans	0	0	0	382
Other
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	925		925		914
with specific allowance, Recorded investment	117		117		121
Total Recorded investment	1,042		1,042		1,035
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	901		901		902
With specific allowance, Unpaid principal balance	116		116		120
Total Unpaid principal balance	1,017		1,017		1,022
Associated Allowance	1		1		0
Recorded Investment of Loans on Nonaccrual Status	0		0		0
Average recorded investment in impaired loans	1,041	723	1,039	817
Income recognized on impaired loans	$ 0	$ 0	$ 0	$ 0