Loans and Allowance for Losses and Concentrations of Credit Risk - Allowance for Credit Losses Roll Forward (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses			$ 5,415
Beginning Balance - Reserve for Losses			2,020
Beginning Balance - Total Allowance for Losses	$ 8,104	$ 7,084	7,435	$ 6,563
Provision for loan losses	(270)	(191)	(1,234)	(604)
Provision for/(release of) reserve for losses	114	(222)	60	(114)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(130)	(241)	(130)
Total provision for losses	384	(31)	1,294	490
Ending Balance - Allowance for Loan Losses	6,408		6,408
Ending Balance - Reserve for Losses	2,080		2,080
Ending Balance - Total Allowance for Losses	8,488	6,923	8,488	6,923
Crops [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	3,735	3,111	3,365	2,791
Financing Receivable, Allowance for Credit Losses, Write-downs		0	(228)	0
Total provision for losses	115	103	713	423
Ending Balance - Total Allowance for Losses	3,850	3,214	3,850	3,214
Permanent Plantings [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	2,164	1,144	1,723	931
Financing Receivable, Allowance for Credit Losses, Write-downs		0	0	0
Total provision for losses	162	198	603	411
Ending Balance - Total Allowance for Losses	2,326	1,342	2,326	1,342
Livestock [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	1,234	1,906	1,375	1,781
Financing Receivable, Allowance for Credit Losses, Write-downs		0	(13)	0
Total provision for losses	35	(354)	(93)	(229)
Ending Balance - Total Allowance for Losses	1,269	1,552	1,269	1,552
Part-Time Farm [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	397	447	405	408
Financing Receivable, Allowance for Credit Losses, Write-downs		(130)	0	(130)
Total provision for losses	4	36	(4)	75
Ending Balance - Total Allowance for Losses	401	353	401	353
Agriculture Storage and Processing [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	558	473	533	649
Financing Receivable, Allowance for Credit Losses, Write-downs		0	0	0
Total provision for losses	72	(13)	97	(189)
Ending Balance - Total Allowance for Losses	630	460	630	460
Other Commodity [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses	16	3	34	3
Financing Receivable, Allowance for Credit Losses, Write-downs		0	0	0
Total provision for losses	(4)	(1)	(22)	(1)
Ending Balance - Total Allowance for Losses	12	2	12	2
Allowance for Loan and Lease Losses [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses	6,138	4,893	5,415	4,480
Provision for loan losses	(270)	(191)	(1,234)	(604)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(130)	(241)	(130)
Ending Balance - Allowance for Loan Losses	6,408	4,954	6,408	4,954
Reserve for Off-balance Sheet Activities [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Reserve for Losses	1,966	2,191	2,020	2,083
Provision for/(release of) reserve for losses	114	(222)	60	(114)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0	0	0
Ending Balance - Reserve for Losses	$ 2,080	$ 1,969	$ 2,080	$ 1,969