Loans and Allowance for Losses and Concentrations of Credit Risk - Allowance for Credit Losses Roll Forward (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses				$ 6,866,000					$ 6,866,000
Beginning Balance - Reserve for Losses				6,468,000					6,468,000
Beginning Balance - Total Allowance for Losses				13,334,000				16,890,000	13,334,000	16,890,000	17,516,000
Release of/(provision for) loan losses	462,000	(511,000)	1,583,000	(573,000)	(117,000)	499,000	529,000	(430,000)	961,000	481,000	(3,691,000)
(Release of)/provision for reserve for losses									(2,205,000)	929,000	(1,816,000)
Total (release of)/provision for losses									(3,166,000)	448,000	1,875,000
Financing Receivable, Allowance for Credit Losses, Write-downs									86,000	4,004,000	2,501,000
Financing Receivable, Allowance for Credit Losses, Recovery									45,000
Ending Balance - Allowance for Loan Losses	5,864,000				6,866,000				5,864,000	6,866,000
Ending Balance - Reserve for Losses	4,263,000				6,468,000				4,263,000	6,468,000
Ending Balance - Total Allowance for Losses	10,127,000				13,334,000				10,127,000	13,334,000	16,890,000
Allowance for Loan and Lease Losses, Adjustments, Net											3,200,000
Crops [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				2,124,000				2,589,000	2,124,000	2,589,000
Total (release of)/provision for losses									395,000	(420,000)
Financing Receivable, Allowance for Credit Losses, Write-downs									0	45,000
Financing Receivable, Allowance for Credit Losses, Recovery									0
Ending Balance - Total Allowance for Losses	2,519,000				2,124,000				2,519,000	2,124,000
Permanent Plantings [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				2,186,000				2,316,000	2,186,000	2,316,000
Total (release of)/provision for losses									(72,000)	(130,000)
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0
Financing Receivable, Allowance for Credit Losses, Recovery									45,000
Ending Balance - Total Allowance for Losses	2,159,000				2,186,000				2,159,000	2,186,000
Livestock [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				1,271,000				1,534,000	1,271,000	1,534,000
Total (release of)/provision for losses									209,000	(263,000)
Financing Receivable, Allowance for Credit Losses, Write-downs									57,000	0
Financing Receivable, Allowance for Credit Losses, Recovery									0
Ending Balance - Total Allowance for Losses	1,423,000				1,271,000				1,423,000	1,271,000
Part-Time Farm [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				454,000				784,000	454,000	784,000
Total (release of)/provision for losses									42,000	4,000
Financing Receivable, Allowance for Credit Losses, Write-downs									29,000	334,000
Financing Receivable, Allowance for Credit Losses, Recovery									0
Ending Balance - Total Allowance for Losses	467,000				454,000				467,000	454,000
Agriculture Storage and Processing [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				7,292,000				9,661,000	7,292,000	9,661,000
Total (release of)/provision for losses									(3,740,000)	1,256,000
Financing Receivable, Allowance for Credit Losses, Write-downs									0	3,625,000
Financing Receivable, Allowance for Credit Losses, Recovery									0
Ending Balance - Total Allowance for Losses	3,552,000				7,292,000				3,552,000	7,292,000
Other Commodity [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				7,000				6,000	7,000	6,000
Total (release of)/provision for losses									0	1,000
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0
Financing Receivable, Allowance for Credit Losses, Recovery									0
Ending Balance - Total Allowance for Losses	7,000				7,000				7,000	7,000
Allowance for Loan and Lease Losses [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses				6,866,000				11,351,000	6,866,000	11,351,000	10,161,000
Release of/(provision for) loan losses									961,000	481,000	(3,691,000)
Financing Receivable, Allowance for Credit Losses, Write-downs									86,000	4,004,000	2,501,000
Ending Balance - Allowance for Loan Losses	5,864,000				6,866,000				5,864,000	6,866,000	11,351,000
Reserve for Off-balance Sheet Activities [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Reserve for Losses				6,468,000				5,539,000	6,468,000	5,539,000	7,355,000
(Release of)/provision for reserve for losses									(2,205,000)	929,000	(1,816,000)
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0	0
Financing Receivable, Allowance for Credit Losses, Recovery									0
Ending Balance - Reserve for Losses	4,263,000				6,468,000				4,263,000	6,468,000	5,539,000
Eligible Loans [Member] | Agriculture Storage and Processing [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Financing Receivable, Allowance for Credit Losses, Write-downs										$ 3,600,000