Loans and Allowance for Losses and Concentrations of Credit Risk - Impaired Financing Receivables (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	$ 35,205		$ 31,457
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	35,135		31,460
Impaired Financing Receivable, with Related Allowance, Recorded Investment	112,288		115,614
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	112,097		115,623
Impaired Financing Receivable, Recorded Investment	147,493		147,071
Impaired Financing Receivable, Unpaid Principal Balance	147,232		147,083
Impaired Financing Receivable, Related Allowance	2,708		2,884
Financing Receivable, Recorded Investment, Nonaccrual Status	64,312		63,593
Impaired Financing Receivable, Average Recorded Investment	147,284	$ 112,021
Impaired Financing Receivable, Interest Income, Cash Basis Method	698	734
Crops [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	18,940		14,417
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	18,902		14,418
Impaired Financing Receivable, with Related Allowance, Recorded Investment	48,367		62,309
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	48,290		62,314
Impaired Financing Receivable, Recorded Investment	67,307		76,726
Impaired Financing Receivable, Unpaid Principal Balance	67,192		76,732
Impaired Financing Receivable, Related Allowance	1,021		1,431
Financing Receivable, Recorded Investment, Nonaccrual Status	28,211		27,630
Impaired Financing Receivable, Average Recorded Investment	72,017	58,309
Impaired Financing Receivable, Interest Income, Cash Basis Method	392	302
Permanent Plantings [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	4,629		3,272
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	4,620		3,273
Impaired Financing Receivable, with Related Allowance, Recorded Investment	41,809		37,143
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	41,731		37,146
Impaired Financing Receivable, Recorded Investment	46,438		40,415
Impaired Financing Receivable, Unpaid Principal Balance	46,351		40,419
Impaired Financing Receivable, Related Allowance	1,227		1,063
Financing Receivable, Recorded Investment, Nonaccrual Status	24,870		25,701
Impaired Financing Receivable, Average Recorded Investment	43,427	31,806
Impaired Financing Receivable, Interest Income, Cash Basis Method	172	152
Livestock [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	8,090		11,171
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	8,074		11,172
Impaired Financing Receivable, with Related Allowance, Recorded Investment	15,910		9,376
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	15,884		9,376
Impaired Financing Receivable, Recorded Investment	24,000		20,547
Impaired Financing Receivable, Unpaid Principal Balance	23,958		20,548
Impaired Financing Receivable, Related Allowance	336		242
Financing Receivable, Recorded Investment, Nonaccrual Status	5,840		5,333
Impaired Financing Receivable, Average Recorded Investment	22,274	13,658
Impaired Financing Receivable, Interest Income, Cash Basis Method	79	177
Part-Time Farm [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,909		1,953
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	2,904		1,953
Impaired Financing Receivable, with Related Allowance, Recorded Investment	6,128		6,710
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	6,118		6,711
Impaired Financing Receivable, Recorded Investment	9,037		8,663
Impaired Financing Receivable, Unpaid Principal Balance	9,022		8,664
Impaired Financing Receivable, Related Allowance	122		146
Financing Receivable, Recorded Investment, Nonaccrual Status	5,391		4,929
Impaired Financing Receivable, Average Recorded Investment	8,850	8,248
Impaired Financing Receivable, Interest Income, Cash Basis Method	55	103
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	637		644
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	635		644
Impaired Financing Receivable, with Related Allowance, Recorded Investment	74		76
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	74		76
Impaired Financing Receivable, Recorded Investment	711		720
Impaired Financing Receivable, Unpaid Principal Balance	709		720
Impaired Financing Receivable, Related Allowance	2		2
Financing Receivable, Recorded Investment, Nonaccrual Status	0		0
Impaired Financing Receivable, Average Recorded Investment	716	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	109,900		113,200
Impaired Financing Receivable, Related Allowance	$ 2,300		$ 2,700
Impaired Financing Receivable, Impairment analysis performed percent	75.00%		77.00%
Less than 90 days past due [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	$ 20,700		$ 15,700