Loans and Allowance for Losses and Concentrations of Credit Risk - Allowance for Losses by Impairment Method and Commodity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	$ 6,784,770		$ 6,720,503
Financing Receivable, Individually Evaluated for Impairment	147,232		147,083
Financing Receivable, Gross	6,932,002		6,867,586
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	5,748		5,982
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2,708		2,884
Financing Receivable, Allowance for Credit Losses	8,456	$ 7,638	8,866	$ 7,435
Total provision for losses	(410)	444
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(241)
Crops [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	3,612,630		3,581,213
Financing Receivable, Individually Evaluated for Impairment	67,192		76,732
Financing Receivable, Gross	3,679,822		3,657,945
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	2,772		2,650
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,021		1,431
Financing Receivable, Allowance for Credit Losses	3,793	3,562	4,081	3,365
Total provision for losses	(288)	425
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(228)
Permanent Plantings [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	1,306,987		1,327,144
Financing Receivable, Individually Evaluated for Impairment	46,351		40,419
Financing Receivable, Gross	1,353,338		1,367,563
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	1,252		1,406
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,227		1,063
Financing Receivable, Allowance for Credit Losses	2,479	1,870	2,469	1,723
Total provision for losses	10	147
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0
Livestock [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	1,323,352		1,314,410
Financing Receivable, Individually Evaluated for Impairment	23,958		20,548
Financing Receivable, Gross	1,347,310		1,334,958
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	900		969
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	336		242
Financing Receivable, Allowance for Credit Losses	1,236	1,379	1,211	1,375
Total provision for losses	25	17
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(13)
Part-Time Farm [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	460,001		424,964
Financing Receivable, Individually Evaluated for Impairment	9,022		8,664
Financing Receivable, Gross	469,023		433,628
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	291		335
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	122		146
Financing Receivable, Allowance for Credit Losses	413	324	481	405
Total provision for losses	(68)	(81)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0
Agriculture Storage and Processing [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	68,909		58,761
Financing Receivable, Individually Evaluated for Impairment	0		0
Financing Receivable, Gross	68,909		58,761
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	522		606
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Allowance for Credit Losses	522	472	606	533
Total provision for losses	(84)	(61)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0
Other Commodity [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	12,891		14,011
Financing Receivable, Individually Evaluated for Impairment	709		720
Financing Receivable, Gross	13,600		14,731
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	11		16
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2		2
Financing Receivable, Allowance for Credit Losses	13	31	18	$ 34
Total provision for losses	(5)	(3)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0
On-balance sheet [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	4,142,141		4,066,457
Financing Receivable, Individually Evaluated for Impairment	132,218		132,276
Financing Receivable, Gross	4,274,359		4,198,733
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	4,041		4,285
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2,324		2,511
Financing Receivable, Allowance for Credit Losses	6,365		6,796
On-balance sheet [Member] | Crops [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	2,366,184		2,344,821
Financing Receivable, Individually Evaluated for Impairment	58,314		67,828
Financing Receivable, Gross	2,424,498		2,412,649
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	2,158		2,104
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	787		1,207
Financing Receivable, Allowance for Credit Losses	2,945		3,311
On-balance sheet [Member] | Permanent Plantings [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	797,591		794,478
Financing Receivable, Individually Evaluated for Impairment	43,960		38,180
Financing Receivable, Gross	841,551		832,658
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	980		1,101
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,169		1,006
Financing Receivable, Allowance for Credit Losses	2,149		2,107
On-balance sheet [Member] | Livestock [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	659,997		635,768
Financing Receivable, Individually Evaluated for Impairment	21,206		17,766
Financing Receivable, Gross	681,203		653,534
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	642		738
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	268		172
Financing Receivable, Allowance for Credit Losses	910		910
On-balance sheet [Member] | Part-Time Farm [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	298,438		269,337
Financing Receivable, Individually Evaluated for Impairment	8,103		7,858
Financing Receivable, Gross	306,541		277,195
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	245		287
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	100		126
Financing Receivable, Allowance for Credit Losses	345		413
On-balance sheet [Member] | Agriculture Storage and Processing [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	10,926		13,023
Financing Receivable, Individually Evaluated for Impairment	0		0
Financing Receivable, Gross	10,926		13,023
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	10		44
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Allowance for Credit Losses	10		44
On-balance sheet [Member] | Other Commodity [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	9,005		9,030
Financing Receivable, Individually Evaluated for Impairment	635		644
Financing Receivable, Gross	9,640		9,674
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	6		11
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Allowance for Credit Losses	6		11
Off-balance sheet [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	2,642,629		2,654,046
Financing Receivable, Individually Evaluated for Impairment	15,014		14,807
Financing Receivable, Gross	2,657,643		2,668,853
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	1,707		1,697
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	384		373
Financing Receivable, Allowance for Credit Losses	2,091		2,070
Off-balance sheet [Member] | Crops [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	1,246,446		1,236,392
Financing Receivable, Individually Evaluated for Impairment	8,878		8,904
Financing Receivable, Gross	1,255,324		1,245,296
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	614		546
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	234		224
Financing Receivable, Allowance for Credit Losses	848		770
Off-balance sheet [Member] | Permanent Plantings [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	509,396		532,666
Financing Receivable, Individually Evaluated for Impairment	2,391		2,239
Financing Receivable, Gross	511,787		534,905
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	272		305
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	58		57
Financing Receivable, Allowance for Credit Losses	330		362
Off-balance sheet [Member] | Livestock [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	663,355		678,642
Financing Receivable, Individually Evaluated for Impairment	2,752		2,782
Financing Receivable, Gross	666,107		681,424
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	258		231
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	68		70
Financing Receivable, Allowance for Credit Losses	326		301
Off-balance sheet [Member] | Part-Time Farm [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	161,563		155,627
Financing Receivable, Individually Evaluated for Impairment	919		806
Financing Receivable, Gross	162,482		156,433
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	46		48
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	22		20
Financing Receivable, Allowance for Credit Losses	68		68
Off-balance sheet [Member] | Agriculture Storage and Processing [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	57,983		45,738
Financing Receivable, Individually Evaluated for Impairment	0		0
Financing Receivable, Gross	57,983		45,738
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	512		562
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Allowance for Credit Losses	512		562
Off-balance sheet [Member] | Other Commodity [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Collectively Evaluated for Impairment	3,886		4,981
Financing Receivable, Individually Evaluated for Impairment	74		76
Financing Receivable, Gross	3,960		5,057
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	5		5
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2		2
Financing Receivable, Allowance for Credit Losses	7		$ 7
Allowance for Loan and Lease Losses [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses, Write-downs	$ 0	$ (241)