Loans and Allowance for Losses and Concentrations of Credit Risk - Allowance for Credit Losses Roll Forward (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	$ 147,071				$ 107,399				$ 147,071	$ 107,399
Financing Receivable, Recorded Investment, Past Due	48,444				21,038				48,444	21,038
Impaired Financing Receivable, Related Allowance	2,884				2,292				2,884	2,292
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities, Subject To Be Purchased	300				100				300	100
Financing Receivable, Recorded Investment, Nonaccrual Status	63,593				32,142				63,593	32,142
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses				$ 5,415					5,415
Beginning Balance - Reserve for Losses				2,020					2,020
Beginning Balance - Total Allowance for Losses				7,435				$ 6,563	7,435	6,563	$ 10,127
Provision for loan losses	(474)	$ (270)	$ (327)	(637)	(461)	$ (191)	$ (364)	(49)	(1,708)	(1,065)	(2,388)
Provision for/(release of) reserve for losses									50	(63)	(2,180)
Financing Receivable, Allowance for Credit Losses, Write-downs									(327)	(130)	(3,772)
Total provision for losses									1,758	1,002	208
Ending Balance - Allowance for Loan Losses	6,796				5,415				6,796	5,415
Ending Balance - Reserve for Losses	2,070				2,020				2,070	2,020
Ending Balance - Total Allowance for Losses	8,866				7,435				8,866	7,435	6,563
Crops [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	76,726				53,087				76,726	53,087
Financing Receivable, Recorded Investment, Past Due	21,853				13,822				21,853	13,822
Impaired Financing Receivable, Related Allowance	1,431				945				1,431	945
Financing Receivable, Recorded Investment, Nonaccrual Status	27,630				13,405				27,630	13,405
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				3,365				2,791	3,365	2,791	2,519
Financing Receivable, Allowance for Credit Losses, Write-downs									(228)	0	0
Total provision for losses									944	574	272
Ending Balance - Total Allowance for Losses	4,081				3,365				4,081	3,365	2,791
Permanent Plantings [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	40,415				30,431				40,415	30,431
Financing Receivable, Recorded Investment, Past Due	18,833				3,652				18,833	3,652
Impaired Financing Receivable, Related Allowance	1,063				790				1,063	790
Financing Receivable, Recorded Investment, Nonaccrual Status	25,701				10,785				25,701	10,785
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				1,723				931	1,723	931	2,159
Financing Receivable, Allowance for Credit Losses, Write-downs									(70)	0	0
Total provision for losses									816	792	(1,228)
Ending Balance - Total Allowance for Losses	2,469				1,723				2,469	1,723	931
Livestock [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	20,547				15,376				20,547	15,376
Financing Receivable, Recorded Investment, Past Due	3,835				707				3,835	707
Impaired Financing Receivable, Related Allowance	242				399				242	399
Financing Receivable, Recorded Investment, Nonaccrual Status	5,333				2,696				5,333	2,696
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				1,375				1,781	1,375	1,781	1,423
Financing Receivable, Allowance for Credit Losses, Write-downs									(13)	0	0
Total provision for losses									(151)	(406)	358
Ending Balance - Total Allowance for Losses	1,211				1,375				1,211	1,375	1,781
Part-Time Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	8,663				8,505				8,663	8,505
Financing Receivable, Recorded Investment, Past Due	3,923				2,857				3,923	2,857
Impaired Financing Receivable, Related Allowance	146				158				146	158
Financing Receivable, Recorded Investment, Nonaccrual Status	4,929				5,256				4,929	5,256
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				405				408	405	408	467
Financing Receivable, Allowance for Credit Losses, Write-downs									(16)	(130)	(111)
Total provision for losses									92	127	52
Ending Balance - Total Allowance for Losses	481				405				481	405	408
Agriculture Storage and Processing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	0				0				0	0
Financing Receivable, Recorded Investment, Past Due	0				0				0	0
Impaired Financing Receivable, Related Allowance	0				0				0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	0				0				0	0
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				533				649	533	649	3,552
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0	(3,661)
Total provision for losses									73	(116)	758
Ending Balance - Total Allowance for Losses	606				533				606	533	649
Other Commodity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	720				0				720	0
Financing Receivable, Recorded Investment, Past Due	0				0				0	0
Impaired Financing Receivable, Related Allowance	2				0				2	0
Financing Receivable, Recorded Investment, Nonaccrual Status	0				0				0	0
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				34				3	34	3	7
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0	0
Total provision for losses									(16)	31	(4)
Ending Balance - Total Allowance for Losses	18				34				18	34	3
Allowance for Loan and Lease Losses [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses				5,415				4,480	5,415	4,480	5,864
Provision for loan losses									(1,708)	(1,065)	(2,388)
Financing Receivable, Allowance for Credit Losses, Write-downs									(327)	(130)	(3,772)
Ending Balance - Allowance for Loan Losses	6,796				5,415				6,796	5,415	4,480
Reserve for Off-balance Sheet Activities [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Reserve for Losses				$ 2,020				$ 2,083	2,020	2,083	4,263
Provision for/(release of) reserve for losses									50	(63)	(2,180)
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0	0
Ending Balance - Reserve for Losses	$ 2,070				$ 2,020				$ 2,070	$ 2,020	$ 2,083