Loans and Allowance for Losses and Concentrations of Credit Risk - Concentrations of Credit Risk (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 7,045,397	$ 6,867,586
Financing Receivables, Borrower Exposure, Range One [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,427,187	2,379,596
Financing Receivables, Borrower Exposure, Range Two [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,729,196	2,627,617
Financing Receivables, Borrower Exposure, Range Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	908,347	867,574
Financing Receivables, Borrower Exposure, Range Four [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	565,184	584,896
Financing Receivables, Borrower Exposure, Range Five [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	415,483	407,903
Financing Receivables, Original Loan-To-Value Ratio, Range One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 1,314,094	1,322,422
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	$ 1,762,371	1,733,671
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	$ 2,440,299	2,385,605
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	$ 1,235,630	1,150,914
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	$ 268,002	248,799
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	$ 25,001	26,175
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	$ 819,607	740,991
Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,170,739	2,093,213
Mid-North [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,279,960	2,244,094
Mid-South [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	879,945	908,603
Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	305,288	296,264
Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	589,858	584,421
Crops [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	3,757,903	3,657,945
Permanent Plantings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	1,391,841	1,367,563
Livestock [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	1,354,863	1,334,958
Part-Time Farm [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	463,988	433,628
Agriculture Storage and Processing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	68,558	58,761
Other Commodity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 8,244	$ 14,731