Loans and Allowance for Losses and Concentrations of Credit Risk - Impaired Financing Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	$ 31,457	$ 27,468
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	31,460	27,534
Impaired Financing Receivable, with Related Allowance, Recorded Investment	115,614	79,931
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	115,623	80,205
Impaired Financing Receivable, Recorded Investment	147,071	107,399
Impaired Financing Receivable, Unpaid Principal Balance	147,083	107,739
Impaired Financing Receivable, Related Allowance	2,884	2,292
Financing Receivable, Recorded Investment, Nonaccrual Status	63,593	32,142
Impaired Financing Receivable, Average Recorded Investment	133,180	82,803
Impaired Financing Receivable, Interest Income, Cash Basis Method	1,753	1,569
Crops [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	14,417	20,761
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	14,418	20,816
Impaired Financing Receivable, with Related Allowance, Recorded Investment	62,309	32,326
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	62,314	32,381
Impaired Financing Receivable, Recorded Investment	76,726	53,087
Impaired Financing Receivable, Unpaid Principal Balance	76,732	53,197
Impaired Financing Receivable, Related Allowance	1,431	945
Financing Receivable, Recorded Investment, Nonaccrual Status	27,630	13,405
Impaired Financing Receivable, Average Recorded Investment	71,154	33,252
Impaired Financing Receivable, Interest Income, Cash Basis Method	696	136
Permanent Plantings [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	3,272	3,683
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	3,273	3,688
Impaired Financing Receivable, with Related Allowance, Recorded Investment	37,143	26,748
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	37,146	26,817
Impaired Financing Receivable, Recorded Investment	40,415	30,431
Impaired Financing Receivable, Unpaid Principal Balance	40,419	30,505
Impaired Financing Receivable, Related Allowance	1,063	790
Financing Receivable, Recorded Investment, Nonaccrual Status	25,701	10,785
Impaired Financing Receivable, Average Recorded Investment	37,597	24,737
Impaired Financing Receivable, Interest Income, Cash Basis Method	530	866
Livestock [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	11,171	1,054
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	11,172	1,055
Impaired Financing Receivable, with Related Allowance, Recorded Investment	9,376	14,322
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	9,376	14,457
Impaired Financing Receivable, Recorded Investment	20,547	15,376
Impaired Financing Receivable, Unpaid Principal Balance	20,548	15,512
Impaired Financing Receivable, Related Allowance	242	399
Financing Receivable, Recorded Investment, Nonaccrual Status	5,333	2,696
Impaired Financing Receivable, Average Recorded Investment	15,913	14,450
Impaired Financing Receivable, Interest Income, Cash Basis Method	238	238
Part-Time Farm [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,953	1,970
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	1,953	1,975
Impaired Financing Receivable, with Related Allowance, Recorded Investment	6,710	6,535
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	6,711	6,550
Impaired Financing Receivable, Recorded Investment	8,663	8,505
Impaired Financing Receivable, Unpaid Principal Balance	8,664	8,525
Impaired Financing Receivable, Related Allowance	146	158
Financing Receivable, Recorded Investment, Nonaccrual Status	4,929	5,256
Impaired Financing Receivable, Average Recorded Investment	8,135	8,396
Impaired Financing Receivable, Interest Income, Cash Basis Method	289	329
Agriculture Storage and Processing [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0	0
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0	0
Impaired Financing Receivable, Recorded Investment	0	0
Impaired Financing Receivable, Unpaid Principal Balance	0	0
Impaired Financing Receivable, Related Allowance	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Impaired Financing Receivable, Average Recorded Investment	0	1,968
Impaired Financing Receivable, Interest Income, Cash Basis Method	0	0
Other Commodity [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	644	0
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	644	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	76	0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	76	0
Impaired Financing Receivable, Recorded Investment	720	0
Impaired Financing Receivable, Unpaid Principal Balance	720	0
Impaired Financing Receivable, Related Allowance	2	0
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Impaired Financing Receivable, Average Recorded Investment	381	0
Impaired Financing Receivable, Interest Income, Cash Basis Method	0	0
Collateral Dependent Not Individually Analyzed [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	113,200	76,500
Impaired Financing Receivable, Related Allowance	$ 2,700	$ 1,600
Impaired Financing Receivable, Impairment analysis performed percent	77.00%	71.00%
Less than 90 days past due [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	$ 15,700	$ 12,400