Loans and Allowance for Losses and Concentrations of Credit Risk - Concentrations of Credit Risk (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 6,867,586	$ 6,139,304
Financing Receivables, Borrower Exposure, Range One [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,379,596	2,195,103
Financing Receivables, Borrower Exposure, Range Two [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,627,617	2,398,843
Financing Receivables, Borrower Exposure, Range Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	867,574	782,171
Financing Receivables, Borrower Exposure, Range Four [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	584,896	469,681
Financing Receivables, Borrower Exposure, Range Five [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	407,903	293,506
Financing Receivables, Original Loan-To-Value Ratio, Range One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 1,322,422	1,220,432
Financing Receivables, Loan-To-Value Ratio Range, Lower Range Limit	0.00%
Financing Receivables, Loan-To-Value Ratio Range, Upper Range Limit	40.00%
Financing Receivables, Original Loan-To-Value Ratio, Range Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 1,733,671	1,466,047
Financing Receivables, Loan-To-Value Ratio Range, Lower Range Limit	40.01%
Financing Receivables, Loan-To-Value Ratio Range, Upper Range Limit	50.00%
Financing Receivables, Original Loan-To-Value Ratio, Range Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 2,385,605	2,078,099
Financing Receivables, Loan-To-Value Ratio Range, Lower Range Limit	50.01%
Financing Receivables, Loan-To-Value Ratio Range, Upper Range Limit	60.00%
Financing Receivables, Original Loan-To-Value Ratio, Range Four [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 1,150,914	1,167,395
Financing Receivables, Loan-To-Value Ratio Range, Lower Range Limit	60.01%
Financing Receivables, Loan-To-Value Ratio Range, Upper Range Limit	70.00%
Financing Receivables, Original Loan-To-Value Ratio, Range Five [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 248,799	191,664
Financing Receivables, Loan-To-Value Ratio Range, Lower Range Limit	70.01%
Financing Receivables, Loan-To-Value Ratio Range, Upper Range Limit	80.00%
Financing Receivables, Original Loan-To-Value Ratio, Range Six [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 26,175	15,667
Financing Receivables, Loan-To-Value Ratio Range, Lower Range Limit	80.01%
Financing Receivables, Loan-To-Value Ratio Range, Upper Range Limit	90.00%
Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 740,991	657,403
Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,093,213	1,791,745
Mid-North [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,244,094	2,104,867
Mid-South [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	908,603	837,121
Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	296,264	229,679
Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	584,421	518,489
Crops [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	3,657,945	3,410,498
Permanent Plantings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	1,367,563	1,037,440
Livestock [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	1,334,958	1,305,844
Part-Time Farm [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	433,628	324,074
Agriculture Storage and Processing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	58,761	48,051
Other Commodity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 14,731	$ 13,397