Loans and Allowance for Losses and Concentrations of Credit Risk - Allowance for Credit Losses Roll Forward (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2015	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	$ 147,493,000			$ 147,071,000
Financing Receivable, Recorded Investment, Past Due	47,560,000			48,444,000
Impaired Financing Receivable, Related Allowance	2,708,000			2,884,000
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities, Subject To Be Purchased	0			300,000
Financing Receivable, Recorded Investment, Nonaccrual Status	64,312,000			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	8,866,000	$ 7,435,000
Release of/(provision for) loan losses	431,000	(637,000)	$ (2,388,000)
Provision for/(release of) reserve for losses	21,000	(193,000)	$ (2,180,000)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(241,000)
Total provision for losses	(410,000)	444,000
Ending Balance - Allowance for Loan Losses	6,365,000
Ending Balance - Reserve for Losses	2,091,000
Ending Balance - Total Allowance for Losses	8,456,000	7,638,000
Crops [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	67,307,000			76,726,000
Financing Receivable, Recorded Investment, Past Due	24,561,000			21,853,000
Impaired Financing Receivable, Related Allowance	1,021,000			1,431,000
Financing Receivable, Recorded Investment, Nonaccrual Status	28,211,000			27,630,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	4,081,000	3,365,000
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(228,000)
Total provision for losses	(288,000)	425,000
Ending Balance - Total Allowance for Losses	3,793,000	3,562,000
Permanent Plantings [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	46,438,000			40,415,000
Financing Receivable, Recorded Investment, Past Due	9,140,000			18,833,000
Impaired Financing Receivable, Related Allowance	1,227,000			1,063,000
Financing Receivable, Recorded Investment, Nonaccrual Status	24,870,000			25,701,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	2,469,000	1,723,000
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0
Total provision for losses	10,000	147,000
Ending Balance - Total Allowance for Losses	2,479,000	1,870,000
Livestock [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	24,000,000			20,547,000
Financing Receivable, Recorded Investment, Past Due	9,351,000			3,835,000
Impaired Financing Receivable, Related Allowance	336,000			242,000
Financing Receivable, Recorded Investment, Nonaccrual Status	5,840,000			5,333,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	1,211,000	1,375,000
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(13,000)
Total provision for losses	25,000	17,000
Ending Balance - Total Allowance for Losses	1,236,000	1,379,000
Part-Time Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	9,037,000			8,663,000
Financing Receivable, Recorded Investment, Past Due	3,872,000			3,923,000
Impaired Financing Receivable, Related Allowance	122,000			146,000
Financing Receivable, Recorded Investment, Nonaccrual Status	5,391,000			4,929,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	481,000	405,000
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0
Total provision for losses	(68,000)	(81,000)
Ending Balance - Total Allowance for Losses	413,000	324,000
Agriculture Storage and Processing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	0			0
Financing Receivable, Recorded Investment, Past Due	0			0
Impaired Financing Receivable, Related Allowance	0			0
Financing Receivable, Recorded Investment, Nonaccrual Status	0			0
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	606,000	533,000
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0
Total provision for losses	(84,000)	(61,000)
Ending Balance - Total Allowance for Losses	522,000	472,000
Other Commodity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	711,000			720,000
Financing Receivable, Recorded Investment, Past Due	636,000			0
Impaired Financing Receivable, Related Allowance	2,000			2,000
Financing Receivable, Recorded Investment, Nonaccrual Status	0			$ 0
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	18,000	34,000
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0
Total provision for losses	(5,000)	(3,000)
Ending Balance - Total Allowance for Losses	13,000	31,000
Allowance for Loan and Lease Losses [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses	6,796,000	5,415,000
Release of/(provision for) loan losses	431,000	(637,000)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(241,000)
Ending Balance - Allowance for Loan Losses	6,365,000	5,811,000
Reserve for Off-balance Sheet Activities [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Reserve for Losses	2,070,000	2,020,000
Provision for/(release of) reserve for losses	21,000	(193,000)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0
Ending Balance - Reserve for Losses	$ 2,091,000	$ 1,827,000