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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses				$ 5,864,000					$ 5,864,000
Beginning Balance - Reserve for Losses				4,263,000					4,263,000
Beginning Balance - Total Allowance for Losses				10,127,000				$ 13,334,000	10,127,000	$ 13,334,000	$ 16,890,000
(Provision for)/release of allowance for loan losses	$ (3,366,000)	$ 1,164,000	$ (110,000)	(76,000)	$ 462,000	$ (511,000)	$ 1,583,000	(573,000)	(2,388,000)	961,000	481,000
(Release of)/provision for reserve for losses									(2,180,000)	(2,205,000)	929,000
Total (release of)/provision for losses									208,000	(3,166,000)	448,000
Financing Receivable, Allowance for Credit Losses, Write-downs									(3,772,000)	(86,000)	(4,004,000)
Financing Receivable, Allowance for Credit Losses, Recovery										45,000
Ending Balance - Allowance for Loan Losses	4,480,000				5,864,000				4,480,000	5,864,000
Ending Balance - Reserve for Losses	2,083,000				4,263,000				2,083,000	4,263,000
Ending Balance - Total Allowance for Losses	6,563,000				10,127,000				6,563,000	10,127,000	13,334,000
Crops [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				2,519,000				2,124,000	2,519,000	2,124,000
Total (release of)/provision for losses									272,000	395,000
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0
Financing Receivable, Allowance for Credit Losses, Recovery										0
Ending Balance - Total Allowance for Losses	2,791,000				2,519,000				2,791,000	2,519,000	2,124,000
Permanent Plantings [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				2,159,000				2,186,000	2,159,000	2,186,000
Total (release of)/provision for losses									(1,228,000)	(72,000)
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	931,000				2,159,000				931,000	2,159,000	2,186,000
Livestock [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				1,423,000				1,271,000	1,423,000	1,271,000
Total (release of)/provision for losses									358,000	209,000
Financing Receivable, Allowance for Credit Losses, Write-downs									0	(57,000)
Financing Receivable, Allowance for Credit Losses, Recovery										0
Ending Balance - Total Allowance for Losses	1,781,000				1,423,000				1,781,000	1,423,000	1,271,000
Part-Time Farm [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				467,000				454,000	467,000	454,000
Total (release of)/provision for losses									52,000	42,000
Financing Receivable, Allowance for Credit Losses, Write-downs									(111,000)	(29,000)
Financing Receivable, Allowance for Credit Losses, Recovery										0
Ending Balance - Total Allowance for Losses	408,000				467,000				408,000	467,000	454,000
Agriculture Storage and Processing [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				3,552,000				7,292,000	3,552,000	7,292,000
Total (release of)/provision for losses									758,000	(3,740,000)
Financing Receivable, Allowance for Credit Losses, Write-downs										0
Financing Receivable, Allowance for Credit Losses, Recovery										0
Ending Balance - Total Allowance for Losses	649,000				3,552,000				649,000	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				7,000	7,000	7,000
Total (release of)/provision for losses									(4,000)	0
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0
Financing Receivable, Allowance for Credit Losses, Recovery										0
Ending Balance - Total Allowance for Losses	3,000				7,000				3,000	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				5,864,000				6,866,000	5,864,000	6,866,000	11,351,000
(Provision for)/release of allowance for loan losses									(2,388,000)	961,000	481,000
Financing Receivable, Allowance for Credit Losses, Write-downs									(3,661,000)	(86,000)	(4,004,000)
Ending Balance - Allowance for Loan Losses	4,480,000				5,864,000				4,480,000	5,864,000	6,866,000
Reserve for Off-balance Sheet Activities [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Reserve for Losses				$ 4,263,000				$ 6,468,000	4,263,000	6,468,000	5,539,000
(Release of)/provision for reserve for losses									(2,180,000)	(2,205,000)	929,000
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0	0
Financing Receivable, Allowance for Credit Losses, Recovery										0
Ending Balance - Reserve for Losses	$ 2,083,000				$ 4,263,000				$ 2,083,000	$ 4,263,000	6,468,000
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)