Loans and Allowance for Losses and Concentrations of Credit Risk - Impaired Financing Receivables (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	$ 36,679		$ 36,679		$ 31,457
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	36,698		36,698		31,460
Impaired Financing Receivable, with Related Allowance, Recorded Investment	125,863		125,863		115,614
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	125,944		125,944		115,623
Impaired Financing Receivable, Recorded Investment	162,542		162,542		147,071
Impaired Financing Receivable, Unpaid Principal Balance	162,642		162,642		147,083
Impaired Financing Receivable, Related Allowance	3,029		3,029		2,884
Financing Receivable, Recorded Investment, Nonaccrual Status	63,103		63,103		63,593
Impaired Financing Receivable, Average Recorded Investment	155,019	$ 119,040	151,170	$ 115,161
Impaired Financing Receivable, Interest Income, Cash Basis Method	941	321	1,639	1,055
Crops [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	13,239		13,239		14,417
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	13,249		13,249		14,418
Impaired Financing Receivable, with Related Allowance, Recorded Investment	63,535		63,535		62,309
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	63,575		63,575		62,314
Impaired Financing Receivable, Recorded Investment	76,774		76,774		76,726
Impaired Financing Receivable, Unpaid Principal Balance	76,824		76,824		76,732
Impaired Financing Receivable, Related Allowance	1,243		1,243		1,431
Financing Receivable, Recorded Investment, Nonaccrual Status	27,442		27,442		27,630
Impaired Financing Receivable, Average Recorded Investment	72,041	65,295	74,527	61,226
Impaired Financing Receivable, Interest Income, Cash Basis Method	327	160	719	462
Permanent Plantings [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	12,564		12,564		3,272
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	12,565		12,565		3,273
Impaired Financing Receivable, with Related Allowance, Recorded Investment	40,835		40,835		37,143
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	40,865		40,865		37,146
Impaired Financing Receivable, Recorded Investment	53,399		53,399		40,415
Impaired Financing Receivable, Unpaid Principal Balance	53,430		53,430		40,419
Impaired Financing Receivable, Related Allowance	1,268		1,268		1,063
Financing Receivable, Recorded Investment, Nonaccrual Status	26,066		26,066		25,701
Impaired Financing Receivable, Average Recorded Investment	49,919	33,222	45,945	32,292
Impaired Financing Receivable, Interest Income, Cash Basis Method	492	68	664	220
Livestock [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	7,816		7,816		11,171
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	7,822		7,822		11,172
Impaired Financing Receivable, with Related Allowance, Recorded Investment	15,089		15,089		9,376
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	15,096		15,096		9,376
Impaired Financing Receivable, Recorded Investment	22,905		22,905		20,547
Impaired Financing Receivable, Unpaid Principal Balance	22,918		22,918		20,548
Impaired Financing Receivable, Related Allowance	369		369		242
Financing Receivable, Recorded Investment, Nonaccrual Status	5,142		5,142		5,333
Impaired Financing Receivable, Average Recorded Investment	23,453	12,557	21,361	13,497
Impaired Financing Receivable, Interest Income, Cash Basis Method	60	22	139	199
Part-Time Farm [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	3,060		3,060		1,953
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	3,062		3,062		1,953
Impaired Financing Receivable, with Related Allowance, Recorded Investment	6,331		6,331		6,710
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	6,335		6,335		6,711
Impaired Financing Receivable, Recorded Investment	9,391		9,391		8,663
Impaired Financing Receivable, Unpaid Principal Balance	9,397		9,397		8,664
Impaired Financing Receivable, Related Allowance	123		123		146
Financing Receivable, Recorded Investment, Nonaccrual Status	4,453		4,453		4,929
Impaired Financing Receivable, Average Recorded Investment	9,214	7,926	8,780	8,119
Impaired Financing Receivable, Interest Income, Cash Basis Method	62	71	117	174
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	73		73		76
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	73		73		76
Impaired Financing Receivable, Recorded Investment	73		73		720
Impaired Financing Receivable, Unpaid Principal Balance	73		73		720
Impaired Financing Receivable, Related Allowance	26		26		2
Financing Receivable, Recorded Investment, Nonaccrual Status	0		0		0
Impaired Financing Receivable, Average Recorded Investment	392	40	557	27
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	123,500		123,500		113,200
Impaired Financing Receivable, Related Allowance	$ 2,600		$ 2,600		$ 2,700
Impaired Financing Receivable, Impairment analysis performed percent	76.00%		76.00%		77.00%
Less than 90 days past due [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	$ 27,400		$ 27,400		$ 15,700