Loans and Allowance for Losses and Concentrations of Credit Risk - Impaired Financing Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	$ 32,012	$ 31,457
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	31,854	31,460
Impaired Financing Receivable, with Related Allowance, Recorded Investment	123,300	115,614
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	122,786	115,623
Impaired Financing Receivable, Recorded Investment	155,312	147,071
Impaired Financing Receivable, Unpaid Principal Balance	154,640	147,083
Impaired Financing Receivable, Related Allowance	3,405	2,884
Financing Receivable, Recorded Investment, Nonaccrual Status	61,408	63,593
Impaired Financing Receivable, Average Recorded Investment	152,777	133,180
Impaired Financing Receivable, Interest Income, Cash Basis Method	3,446	1,753
Crops [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	20,734	14,417
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	20,632	14,418
Impaired Financing Receivable, with Related Allowance, Recorded Investment	59,335	62,309
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	59,098	62,314
Impaired Financing Receivable, Recorded Investment	80,069	76,726
Impaired Financing Receivable, Unpaid Principal Balance	79,730	76,732
Impaired Financing Receivable, Related Allowance	1,606	1,431
Financing Receivable, Recorded Investment, Nonaccrual Status	26,611	27,630
Impaired Financing Receivable, Average Recorded Investment	74,804	71,154
Impaired Financing Receivable, Interest Income, Cash Basis Method	1,219	696
Permanent Plantings [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	3,592	3,272
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	3,573	3,273
Impaired Financing Receivable, with Related Allowance, Recorded Investment	38,176	37,143
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	38,009	37,146
Impaired Financing Receivable, Recorded Investment	41,768	40,415
Impaired Financing Receivable, Unpaid Principal Balance	41,582	40,419
Impaired Financing Receivable, Related Allowance	1,134	1,063
Financing Receivable, Recorded Investment, Nonaccrual Status	21,349	25,701
Impaired Financing Receivable, Average Recorded Investment	44,461	37,597
Impaired Financing Receivable, Interest Income, Cash Basis Method	1,687	530
Livestock [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	5,764	11,171
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	5,737	11,172
Impaired Financing Receivable, with Related Allowance, Recorded Investment	19,443	9,376
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	19,361	9,376
Impaired Financing Receivable, Recorded Investment	25,207	20,547
Impaired Financing Receivable, Unpaid Principal Balance	25,098	20,548
Impaired Financing Receivable, Related Allowance	522	242
Financing Receivable, Recorded Investment, Nonaccrual Status	8,803	5,333
Impaired Financing Receivable, Average Recorded Investment	24,523	15,913
Impaired Financing Receivable, Interest Income, Cash Basis Method	299	238
Part-Time Farm [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,922	1,953
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	1,912	1,953
Impaired Financing Receivable, with Related Allowance, Recorded Investment	6,276	6,710
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	6,249	6,711
Impaired Financing Receivable, Recorded Investment	8,198	8,663
Impaired Financing Receivable, Unpaid Principal Balance	8,161	8,664
Impaired Financing Receivable, Related Allowance	142	146
Financing Receivable, Recorded Investment, Nonaccrual Status	4,645	4,929
Impaired Financing Receivable, Average Recorded Investment	8,758	8,135
Impaired Financing Receivable, Interest Income, Cash Basis Method	241	289
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	0
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	0	644
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	0	644
Impaired Financing Receivable, with Related Allowance, Recorded Investment	70	76
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	69	76
Impaired Financing Receivable, Recorded Investment	70	720
Impaired Financing Receivable, Unpaid Principal Balance	69	720
Impaired Financing Receivable, Related Allowance	1	2
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Impaired Financing Receivable, Average Recorded Investment	231	381
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	120,900	113,200
Impaired Financing Receivable, Related Allowance	$ 2,700	$ 2,700
Impaired Financing Receivable, Impairment analysis performed percent	78.00%	77.00%
Less than 90 days past due [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	$ 41,800	$ 15,700