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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Change in Provision									$ (1,400)
Impaired Financing Receivable, Recorded Investment	$ 155,312				$ 147,071				155,312	$ 147,071
Financing Receivable, Recorded Investment, Past Due	26,881				48,444				26,881	48,444
Impaired Financing Receivable, Related Allowance	3,405				2,884				3,405	2,884
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities, Subject To Be Purchased	100				300				100	300
Financing Receivable, Recorded Investment, Nonaccrual Status	61,408				63,593				61,408	63,593
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses				$ 6,796					6,796
Beginning Balance - Reserve for Losses				2,070					2,070
Beginning Balance - Total Allowance for Losses				8,866				$ 7,435	8,866	7,435	$ 6,563
Provision for loan losses	(146)	$ (99)	$ (424)	431	(474)	$ (270)	$ (327)	(637)	(238)	(1,708)	(1,065)
Provision for/(release of) reserve for losses									97	50	(63)
Financing Receivable, Allowance for Credit Losses, Write-downs									(17)	(327)	(130)
Total provision for losses									335	1,758	1,002
Ending Balance - Allowance for Loan Losses	7,017				6,796				7,017	6,796
Ending Balance - Reserve for Losses	2,167				2,070				2,167	2,070
Ending Balance - Total Allowance for Losses	9,184				8,866				9,184	8,866	7,435
Crops [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	80,069				76,726				80,069	76,726
Financing Receivable, Recorded Investment, Past Due	14,821				21,853				14,821	21,853
Impaired Financing Receivable, Related Allowance	1,606				1,431				1,606	1,431
Financing Receivable, Recorded Investment, Nonaccrual Status	26,611				27,630				26,611	27,630
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				4,081				3,365	4,081	3,365	2,791
Financing Receivable, Allowance for Credit Losses, Write-downs									0	228	0
Total provision for losses									313	944	574
Ending Balance - Total Allowance for Losses	4,394				4,081				4,394	4,081	3,365
Permanent Plantings [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	41,768				40,415				41,768	40,415
Financing Receivable, Recorded Investment, Past Due	3,194				18,833				3,194	18,833
Impaired Financing Receivable, Related Allowance	1,134				1,063				1,134	1,063
Financing Receivable, Recorded Investment, Nonaccrual Status	21,349				25,701				21,349	25,701
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				2,469				1,723	2,469	1,723	931
Financing Receivable, Allowance for Credit Losses, Write-downs									0	70	0
Total provision for losses									(343)	816	792
Ending Balance - Total Allowance for Losses	2,126				2,469				2,126	2,469	1,723
Livestock [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	25,207				20,547				25,207	20,547
Financing Receivable, Recorded Investment, Past Due	4,059				3,835				4,059	3,835
Impaired Financing Receivable, Related Allowance	522				242				522	242
Financing Receivable, Recorded Investment, Nonaccrual Status	8,803				5,333				8,803	5,333
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				1,211				1,375	1,211	1,375	1,781
Financing Receivable, Allowance for Credit Losses, Write-downs									0	13	0
Total provision for losses									249	(151)	(406)
Ending Balance - Total Allowance for Losses	1,460				1,211				1,460	1,211	1,375
Part-Time Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	8,198				8,663				8,198	8,663
Financing Receivable, Recorded Investment, Past Due	4,807				3,923				4,807	3,923
Impaired Financing Receivable, Related Allowance	142				146				142	146
Financing Receivable, Recorded Investment, Nonaccrual Status	4,645				4,929				4,645	4,929
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				481				405	481	405	408
Financing Receivable, Allowance for Credit Losses, Write-downs									17	16	(130)
Total provision for losses									10	92	127
Ending Balance - Total Allowance for Losses	474				481				474	481	405
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				606				533	606	533	649
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0	0
Total provision for losses									114	73	(116)
Ending Balance - Total Allowance for Losses	720				606				720	606	533
Other Commodity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	70				720				70	720
Financing Receivable, Recorded Investment, Past Due	0				0				0	0
Impaired Financing Receivable, Related Allowance	1				2				1	2
Financing Receivable, Recorded Investment, Nonaccrual Status	0				0				0	0
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Total Allowance for Losses				18				34	18	34	3
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0	0
Total provision for losses									(8)	(16)	31
Ending Balance - Total Allowance for Losses	10				18				10	18	34
SEC Schedule, 12-09, Allowance, Loan and Lease Loss [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Allowance for Loan Losses				6,796				5,415	6,796	5,415	4,480
Provision for loan losses									(238)	(1,708)	(1,065)
Financing Receivable, Allowance for Credit Losses, Write-downs									(17)	(327)	(130)
Ending Balance - Allowance for Loan Losses	7,017				6,796				7,017	6,796	5,415
SEC Schedule, 12-09, Reserve, Off-balance Sheet Activity [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance - Reserve for Losses				$ 2,070				$ 2,020	2,070	2,020	2,083
Provision for loan losses									(97)
Provision for/(release of) reserve for losses										50	(63)
Financing Receivable, Allowance for Credit Losses, Write-downs									0	0	0
Ending Balance - Reserve for Losses	$ 2,167				$ 2,070				$ 2,167	$ 2,070	$ 2,020