Loans and Allowance for Losses and Concentrations of Credit Risk - Concentrations of Credit Risk (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 6,932,002	$ 6,867,586
Financing Receivables, Borrower Exposure, Range One [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,404,191	2,379,596
Financing Receivables, Borrower Exposure, Range Two [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,663,554	2,627,617
Financing Receivables, Borrower Exposure, Range Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	887,986	867,574
Financing Receivables, Borrower Exposure, Range Four [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	571,315	584,896
Financing Receivables, Borrower Exposure, Range Five [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	404,956	407,903
Financing Receivables, Original Loan-To-Value Ratio, Range One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 1,328,529	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	$ 1,717,932	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	$ 2,395,568	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	$ 1,195,017	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	$ 269,716	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	$ 25,240	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	$ 799,823	740,991
Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,096,948	2,093,213
Mid-North [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	2,249,763	2,244,094
Mid-South [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	901,231	908,603
Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	300,731	296,264
Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	583,506	584,421
Crops [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	3,679,822	3,657,945
Permanent Plantings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	1,353,338	1,367,563
Livestock [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	1,347,310	1,334,958
Part-Time Farm [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	469,023	433,628
Agriculture Storage and Processing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	68,909	58,761
Other Commodity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Gross	$ 13,600	$ 14,731