Loans and Allowance for Losses and Concentrations of Credit Risk - Allowance for Credit Losses Roll Forward (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	$ 162,542,000		$ 162,542,000		$ 147,071,000
Financing Receivable, Recorded Investment, Past Due	43,076,000		43,076,000		48,444,000
Impaired Financing Receivable, Related Allowance	3,029,000		3,029,000		2,884,000
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities, Subject To Be Purchased	700,000		700,000		300,000
Financing Receivable, Recorded Investment, Nonaccrual Status	63,103,000		63,103,000		63,593,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses			6,796,000
Beginning Balance - Reserve for Losses			2,070,000
Beginning Balance - Total Allowance for Losses	8,456,000	$ 7,638,000	8,866,000	$ 7,435,000
(Provision for)/release of loan losses	(424,000)	(327,000)	7,000	(964,000)
Provision for/(release of) reserve for losses	158,000	139,000	179,000	(54,000)
Financing Receivable, Allowance for Credit Losses, Write-downs			0	(241,000)
Total provision for losses	582,000	466,000	172,000	910,000
Ending Balance - Allowance for Loan Losses	6,789,000		6,789,000
Ending Balance - Reserve for Losses	2,249,000		2,249,000
Ending Balance - Total Allowance for Losses	9,038,000	8,104,000	9,038,000	8,104,000
Crops [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	76,774,000		76,774,000		76,726,000
Financing Receivable, Recorded Investment, Past Due	23,037,000		23,037,000		21,853,000
Impaired Financing Receivable, Related Allowance	1,243,000		1,243,000		1,431,000
Financing Receivable, Recorded Investment, Nonaccrual Status	27,442,000		27,442,000		27,630,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	3,793,000	3,562,000	4,081,000	3,365,000
Financing Receivable, Allowance for Credit Losses, Write-downs				(228,000)
Total provision for losses	332,000	173,000	44,000	598,000
Ending Balance - Total Allowance for Losses	4,125,000	3,735,000	4,125,000	3,735,000
Permanent Plantings [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	53,399,000		53,399,000		40,415,000
Financing Receivable, Recorded Investment, Past Due	8,363,000		8,363,000		18,833,000
Impaired Financing Receivable, Related Allowance	1,268,000		1,268,000		1,063,000
Financing Receivable, Recorded Investment, Nonaccrual Status	26,066,000		26,066,000		25,701,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	2,479,000	1,870,000	2,469,000	1,723,000
Financing Receivable, Allowance for Credit Losses, Write-downs				0
Total provision for losses	(111,000)	294,000	(101,000)	441,000
Ending Balance - Total Allowance for Losses	2,368,000	2,164,000	2,368,000	2,164,000
Livestock [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	22,905,000		22,905,000		20,547,000
Financing Receivable, Recorded Investment, Past Due	7,739,000		7,739,000		3,835,000
Impaired Financing Receivable, Related Allowance	369,000		369,000		242,000
Financing Receivable, Recorded Investment, Nonaccrual Status	5,142,000		5,142,000		5,333,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	1,236,000	1,379,000	1,211,000	1,375,000
Financing Receivable, Allowance for Credit Losses, Write-downs				(13,000)
Total provision for losses	86,000	(145,000)	111,000	(128,000)
Ending Balance - Total Allowance for Losses	1,322,000	1,234,000	1,322,000	1,234,000
Part-Time Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	9,391,000		9,391,000		8,663,000
Financing Receivable, Recorded Investment, Past Due	3,937,000		3,937,000		3,923,000
Impaired Financing Receivable, Related Allowance	123,000		123,000		146,000
Financing Receivable, Recorded Investment, Nonaccrual Status	4,453,000		4,453,000		4,929,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	413,000	324,000	481,000	405,000
Financing Receivable, Allowance for Credit Losses, Write-downs				0
Total provision for losses	35,000	73,000	(33,000)	(8,000)
Ending Balance - Total Allowance for Losses	448,000	397,000	448,000	397,000
Agriculture Storage and Processing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	0		0		0
Financing Receivable, Recorded Investment, Past Due	0		0		0
Impaired Financing Receivable, Related Allowance	0		0		0
Financing Receivable, Recorded Investment, Nonaccrual Status	0		0		0
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	522,000	472,000	606,000	533,000
Financing Receivable, Allowance for Credit Losses, Write-downs				0
Total provision for losses	198,000	86,000	114,000	25,000
Ending Balance - Total Allowance for Losses	720,000	558,000	720,000	558,000
Other Commodity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	73,000		73,000		720,000
Financing Receivable, Recorded Investment, Past Due	0		0		0
Impaired Financing Receivable, Related Allowance	26,000		26,000		2,000
Financing Receivable, Recorded Investment, Nonaccrual Status	0		0		$ 0
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	13,000	31,000	18,000	34,000
Financing Receivable, Allowance for Credit Losses, Write-downs				0
Total provision for losses	42,000	(15,000)	37,000	(18,000)
Ending Balance - Total Allowance for Losses	55,000	16,000	55,000	16,000
Allowance for Loan and Lease Losses [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses	6,365,000	5,811,000	6,796,000	5,415,000
(Provision for)/release of loan losses	(424,000)	(327,000)	7,000	(964,000)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0	0	(241,000)
Ending Balance - Allowance for Loan Losses	6,789,000	6,138,000	6,789,000	6,138,000
Reserve for Off-balance Sheet Activities [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Reserve for Losses	2,091,000	1,827,000	2,070,000	2,020,000
Provision for/(release of) reserve for losses	158,000	139,000	179,000	(54,000)
Financing Receivable, Allowance for Credit Losses, Write-downs			0	0
Ending Balance - Reserve for Losses	$ 2,249,000	$ 1,966,000	$ 2,249,000	$ 1,966,000