Allowance for Losses and Concentration of Credit Risk - Impaired Financing Receivables (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	$ 28,214		$ 28,214		$ 24,295
with specific allowance, Recorded investment	82,507		82,507		64,068
Total Recorded investment	110,721		110,721		88,363
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	27,701		27,701		25,691
With specific allowance, Unpaid principal balance	80,803		80,803		62,335
Total Unpaid principal balance	108,504		108,504		88,026
Associated Allowance	8,011		8,011		7,296
Recorded Investment of Loans on Nonaccrual Status	49,773		49,773		45,338
Average recorded investment in impaired loans	105,707	94,185	100,787	90,923
Income recognized on impaired loans	1,257	705	2,364	2,139
Crops
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	7,863		7,863		6,809
with specific allowance, Recorded investment	24,905		24,905		23,009
Total Recorded investment	32,768		32,768		29,818
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	7,752		7,752		7,446
With specific allowance, Unpaid principal balance	24,034		24,034		22,074
Total Unpaid principal balance	31,786		31,786		29,520
Associated Allowance	2,350		2,350		2,410
Recorded Investment of Loans on Nonaccrual Status	13,533		13,533		9,214
Average recorded investment in impaired loans	31,490	31,639	29,583	30,546
Income recognized on impaired loans	72	120	213	432
Permanent Plantings
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	10,060		10,060		10,083
with specific allowance, Recorded investment	28,302		28,302		18,668
Total Recorded investment	38,362		38,362		28,751
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	10,038		10,038		9,957
With specific allowance, Unpaid principal balance	27,901		27,901		18,288
Total Unpaid principal balance	37,939		37,939		28,245
Associated Allowance	1,680		1,680		2,324
Recorded Investment of Loans on Nonaccrual Status	22,601		22,601		25,710
Average recorded investment in impaired loans	34,566	31,299	34,284	30,070
Income recognized on impaired loans	1,015	480	1,691	857
Livestock
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	6,007		6,007		3,248
with specific allowance, Recorded investment	11,529		11,529		7,036
Total Recorded investment	17,536		17,536		10,284
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	5,686		5,686		4,088
With specific allowance, Unpaid principal balance	11,348		11,348		6,796
Total Unpaid principal balance	17,034		17,034		10,884
Associated Allowance	211		211		407
Recorded Investment of Loans on Nonaccrual Status	5,219		5,219		3,483
Average recorded investment in impaired loans	17,643	12,371	14,973	13,344
Income recognized on impaired loans	94	42	210	343
Part-Time Farm
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	3,377		3,377		3,241
with specific allowance, Recorded investment	13,156		13,156		9,392
Total Recorded investment	16,533		16,533		12,633
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	3,323		3,323		3,298
With specific allowance, Unpaid principal balance	13,068		13,068		9,215
Total Unpaid principal balance	16,391		16,391		12,513
Associated Allowance	569		569		305
Recorded Investment of Loans on Nonaccrual Status	8,420		8,420		6,931
Average recorded investment in impaired loans	16,526	11,511	16,127	9,753
Income recognized on impaired loans	76	63	250	125
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	4,498		4,498		5,842
Total Recorded investment	4,498		4,498		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	3,200		3,200		1,850
Recorded Investment of Loans on Nonaccrual Status	0		0		0
Average recorded investment in impaired loans	4,449	6,158	4,785	6,439
Income recognized on impaired loans	0	0	0	382
Other
Impaired Financing Receivable, Recorded Investment [Abstract]
With no specific allowance, Recorded investment	907		907		914
with specific allowance, Recorded investment	117		117		121
Total Recorded investment	1,024		1,024		1,035
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
With no specific allowance, Unpaid principal balance	902		902		902
With specific allowance, Unpaid principal balance	115		115		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	1,033	1,207	1,035	771
Income recognized on impaired loans	$ 0	$ 0	$ 0	$ 0