Loans and Allowance for Losses and Concentrations of Credit Risk - Impaired Financing Receivables (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	$ 28,130		$ 28,130		$ 31,457
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	28,199		28,199		31,460
Impaired Financing Receivable, with Related Allowance, Recorded Investment	117,628		117,628		115,614
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	117,876		117,876		115,623
Impaired Financing Receivable, Recorded Investment	145,758		145,758		147,071
Impaired Financing Receivable, Unpaid Principal Balance	146,075		146,075		147,083
Impaired Financing Receivable, Related Allowance	3,256		3,256		2,884
Financing Receivable, Recorded Investment, Nonaccrual Status	63,338		63,338		63,593
Impaired Financing Receivable, Average Recorded Investment	154,151	$ 134,503	150,716	$ 123,262
Impaired Financing Receivable, Interest Income, Cash Basis Method	1,334	419	2,973	1,474
Crops [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	14,815		14,815		14,417
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	14,851		14,851		14,418
Impaired Financing Receivable, with Related Allowance, Recorded Investment	60,250		60,250		62,309
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	60,372		60,372		62,314
Impaired Financing Receivable, Recorded Investment	75,065		75,065		76,726
Impaired Financing Receivable, Unpaid Principal Balance	75,223		75,223		76,732
Impaired Financing Receivable, Related Allowance	1,367		1,367		1,431
Financing Receivable, Recorded Investment, Nonaccrual Status	28,098		28,098		27,630
Impaired Financing Receivable, Average Recorded Investment	75,920	72,180	73,968	65,244
Impaired Financing Receivable, Interest Income, Cash Basis Method	223	101	942	563
Permanent Plantings [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	848		848		3,272
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	851		851		3,273
Impaired Financing Receivable, with Related Allowance, Recorded Investment	35,389		35,389		37,143
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	35,471		35,471		37,146
Impaired Financing Receivable, Recorded Investment	36,237		36,237		40,415
Impaired Financing Receivable, Unpaid Principal Balance	36,322		36,322		40,419
Impaired Financing Receivable, Related Allowance	1,262		1,262		1,063
Financing Receivable, Recorded Investment, Nonaccrual Status	23,090		23,090		25,701
Impaired Financing Receivable, Average Recorded Investment	44,818	38,396	44,122	35,101
Impaired Financing Receivable, Interest Income, Cash Basis Method	933	244	1,597	464
Livestock [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	10,431		10,431		11,171
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	10,456		10,456		11,172
Impaired Financing Receivable, with Related Allowance, Recorded Investment	15,550		15,550		9,376
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	15,581		15,581		9,376
Impaired Financing Receivable, Recorded Investment	25,981		25,981		20,547
Impaired Financing Receivable, Unpaid Principal Balance	26,037		26,037		20,548
Impaired Financing Receivable, Related Allowance	494		494		242
Financing Receivable, Recorded Investment, Nonaccrual Status	7,859		7,859		5,333
Impaired Financing Receivable, Average Recorded Investment	24,443	15,582	23,358	14,620
Impaired Financing Receivable, Interest Income, Cash Basis Method	122	13	261	212
Part-Time Farm [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,036		2,036		1,953
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	2,041		2,041		1,953
Impaired Financing Receivable, with Related Allowance, Recorded Investment	6,368		6,368		6,710
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	6,381		6,381		6,711
Impaired Financing Receivable, Recorded Investment	8,404		8,404		8,663
Impaired Financing Receivable, Unpaid Principal Balance	8,422		8,422		8,664
Impaired Financing Receivable, Related Allowance	132		132		146
Financing Receivable, Recorded Investment, Nonaccrual Status	4,291		4,291		4,929
Impaired Financing Receivable, Average Recorded Investment	8,898	7,944	8,874	8,096
Impaired Financing Receivable, Interest Income, Cash Basis Method	56	61	173	235
Agriculture Storage and Processing [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0		0		0
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	0		0		0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0		0
Impaired Financing Receivable, Recorded Investment	0		0		0
Impaired Financing Receivable, Unpaid Principal Balance	0		0		0
Impaired Financing Receivable, Related Allowance	0		0		0
Financing Receivable, Recorded Investment, Nonaccrual Status	0		0		0
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Cash Basis Method	0	0	0	0
Other Commodity [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0		0		644
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	0		0		644
Impaired Financing Receivable, with Related Allowance, Recorded Investment	71		71		76
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	71		71		76
Impaired Financing Receivable, Recorded Investment	71		71		720
Impaired Financing Receivable, Unpaid Principal Balance	71		71		720
Impaired Financing Receivable, Related Allowance	1		1		2
Financing Receivable, Recorded Investment, Nonaccrual Status	0		0		0
Impaired Financing Receivable, Average Recorded Investment	72	401	394	201
Impaired Financing Receivable, Interest Income, Cash Basis Method	0	$ 0	0	$ 0
Collateral Dependent Not Individually Analyzed [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	115,200		115,200		113,200
Impaired Financing Receivable, Related Allowance	$ 2,700		$ 2,700		$ 2,700
Impaired Financing Receivable, Impairment analysis performed percent	79.00%		79.00%		77.00%
Less than 90 days past due [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	$ 30,600		$ 30,600		$ 15,700