Loans and Allowance for Losses and Concentrations of Credit Risk - Financing Receivables Credit Quality Indicators (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 0	$ 177,033
Financing Receivable, Modifications, Pre-Modification Recorded Investment		$ 200,000
Financing Receivable, Gross	7,045,397,000		$ 6,867,586,000
Financing Receivable, Recorded Investment, Past Due	43,076,000		48,444,000
Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	6,535,139,000		6,390,271,000
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	283,718,000		256,008,000
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	226,540,000		221,307,000
Crops [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	3,757,903,000		3,657,945,000
Financing Receivable, Recorded Investment, Past Due	23,037,000		21,853,000
Crops [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	3,462,088,000		3,407,108,000
Crops [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	185,581,000		146,841,000
Crops [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	110,234,000		103,996,000
Permanent Plantings [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	1,391,841,000		1,367,563,000
Financing Receivable, Recorded Investment, Past Due	8,363,000		18,833,000
Permanent Plantings [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	1,292,776,000		1,250,173,000
Permanent Plantings [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	32,124,000		49,012,000
Permanent Plantings [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	66,941,000		68,378,000
Livestock [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	1,354,863,000		1,334,958,000
Financing Receivable, Recorded Investment, Past Due	7,739,000		3,835,000
Livestock [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	1,261,847,000		1,252,160,000
Livestock [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	59,087,000		49,856,000
Livestock [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	33,929,000		32,942,000
Part-Time Farm [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	463,988,000		433,628,000
Financing Receivable, Recorded Investment, Past Due	3,937,000		3,923,000
Part-Time Farm [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	443,722,000		411,760,000
Part-Time Farm [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	6,768,000		10,130,000
Part-Time Farm [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	13,498,000		11,738,000
Agriculture Storage and Processing [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	68,558,000		58,761,000
Financing Receivable, Recorded Investment, Past Due	0		0
Agriculture Storage and Processing [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	67,194,000		55,746,000
Agriculture Storage and Processing [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	0		0
Agriculture Storage and Processing [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	1,364,000		3,015,000
Other Commodity [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	8,244,000		14,731,000
Financing Receivable, Recorded Investment, Past Due	0		0
Other Commodity [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	7,512,000		13,324,000
Other Commodity [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	158,000		169,000
Other Commodity [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	574,000		1,238,000
On-balance sheet [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	4,378,958,000		4,198,733,000
Financing Receivable, Recorded Investment, Past Due	35,744,000		47,881,000
On-balance sheet [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	4,099,028,000		3,946,946,000
On-balance sheet [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	148,916,000		119,829,000
On-balance sheet [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	131,014,000		131,958,000
On-balance sheet [Member] | Crops [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	2,490,792,000		2,412,649,000
Financing Receivable, Recorded Investment, Past Due	17,290,000		21,702,000
On-balance sheet [Member] | Crops [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	2,315,593,000		2,274,912,000
On-balance sheet [Member] | Crops [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	109,471,000		70,063,000
On-balance sheet [Member] | Crops [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	65,728,000		67,674,000
On-balance sheet [Member] | Permanent Plantings [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	877,777,000		832,658,000
Financing Receivable, Recorded Investment, Past Due	8,363,000		18,833,000
On-balance sheet [Member] | Permanent Plantings [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	829,733,000		771,600,000
On-balance sheet [Member] | Permanent Plantings [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	7,350,000		22,878,000
On-balance sheet [Member] | Permanent Plantings [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	40,694,000		38,180,000
On-balance sheet [Member] | Livestock [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	694,388,000		653,534,000
Financing Receivable, Recorded Investment, Past Due	6,654,000		3,835,000
On-balance sheet [Member] | Livestock [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	651,584,000		617,527,000
On-balance sheet [Member] | Livestock [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	26,248,000		18,405,000
On-balance sheet [Member] | Livestock [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	16,556,000		17,602,000
On-balance sheet [Member] | Part-Time Farm [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	300,938,000		277,195,000
Financing Receivable, Recorded Investment, Past Due	3,437,000		3,511,000
On-balance sheet [Member] | Part-Time Farm [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	287,055,000		260,854,000
On-balance sheet [Member] | Part-Time Farm [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	5,847,000		8,483,000
On-balance sheet [Member] | Part-Time Farm [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	8,036,000		7,858,000
On-balance sheet [Member] | Agriculture Storage and Processing [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	10,496,000		13,023,000
Financing Receivable, Recorded Investment, Past Due	0		0
On-balance sheet [Member] | Agriculture Storage and Processing [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	10,496,000		13,023,000
On-balance sheet [Member] | Agriculture Storage and Processing [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	0		0
On-balance sheet [Member] | Agriculture Storage and Processing [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	0		0
On-balance sheet [Member] | Other Commodity [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	4,567,000		9,674,000
Financing Receivable, Recorded Investment, Past Due	0		0
On-balance sheet [Member] | Other Commodity [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	4,567,000		9,030,000
On-balance sheet [Member] | Other Commodity [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	0		0
On-balance sheet [Member] | Other Commodity [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	0		644,000
Off-balance sheet [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	2,666,439,000		2,668,853,000
Financing Receivable, Recorded Investment, Past Due	7,332,000		563,000
Off-balance sheet [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	2,436,111,000		2,443,325,000
Off-balance sheet [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	134,802,000		136,179,000
Off-balance sheet [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	95,526,000		89,349,000
Off-balance sheet [Member] | Crops [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	1,267,111,000		1,245,296,000
Financing Receivable, Recorded Investment, Past Due	5,747,000		151,000
Off-balance sheet [Member] | Crops [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	1,146,495,000		1,132,196,000
Off-balance sheet [Member] | Crops [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	76,110,000		76,778,000
Off-balance sheet [Member] | Crops [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	44,506,000		36,322,000
Off-balance sheet [Member] | Permanent Plantings [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	514,064,000		534,905,000
Financing Receivable, Recorded Investment, Past Due	0		0
Off-balance sheet [Member] | Permanent Plantings [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	463,043,000		478,573,000
Off-balance sheet [Member] | Permanent Plantings [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	24,774,000		26,134,000
Off-balance sheet [Member] | Permanent Plantings [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	26,247,000		30,198,000
Off-balance sheet [Member] | Livestock [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	660,475,000		681,424,000
Financing Receivable, Recorded Investment, Past Due	1,085,000		0
Off-balance sheet [Member] | Livestock [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	610,263,000		634,633,000
Off-balance sheet [Member] | Livestock [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	32,839,000		31,451,000
Off-balance sheet [Member] | Livestock [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	17,373,000		15,340,000
Off-balance sheet [Member] | Part-Time Farm [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	163,050,000		156,433,000
Financing Receivable, Recorded Investment, Past Due	500,000		412,000
Off-balance sheet [Member] | Part-Time Farm [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	156,667,000		150,906,000
Off-balance sheet [Member] | Part-Time Farm [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	921,000		1,647,000
Off-balance sheet [Member] | Part-Time Farm [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	5,462,000		3,880,000
Off-balance sheet [Member] | Agriculture Storage and Processing [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	58,062,000		45,738,000
Financing Receivable, Recorded Investment, Past Due	0		0
Off-balance sheet [Member] | Agriculture Storage and Processing [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	56,698,000		42,723,000
Off-balance sheet [Member] | Agriculture Storage and Processing [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	0		0
Off-balance sheet [Member] | Agriculture Storage and Processing [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	1,364,000		3,015,000
Off-balance sheet [Member] | Other Commodity [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	3,677,000		5,057,000
Financing Receivable, Recorded Investment, Past Due	0		0
Off-balance sheet [Member] | Other Commodity [Member] | Acceptable [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	2,945,000		4,294,000
Off-balance sheet [Member] | Other Commodity [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	158,000		169,000
Off-balance sheet [Member] | Other Commodity [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Gross	$ 574,000		$ 594,000