Loans (Analysis of the Aged Loan Portfolio) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current Loans	$ 8,863,504	$ 8,686,523
Past Due Loans	22,170	23,882
Nonaccrual Loans	47,647	33,060
Total Loans	8,933,321	8,743,465
Commercial Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	3,957,092	3,935,633
Past Due Loans	7,364	6,421
Nonaccrual Loans	5,518	3,799
Total Loans	3,969,974	3,945,853
Commercial Business [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	3,140,629	2,959,826
Past Due Loans	4,644	8,850
Nonaccrual Loans	24,395	20,937
Total Loans	3,169,668	2,989,613
Agriculture [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	735,186	755,719
Past Due Loans	4,222	4,629
Nonaccrual Loans	15,083	5,023
Total Loans	754,491	765,371
Construction [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	305,948	360,582
Past Due Loans	2,238	951
Nonaccrual Loans	0	0
Total Loans	308,186	361,533
One-to-Four Family Residential [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	684,248	631,109
Past Due Loans	3,615	2,924
Nonaccrual Loans	2,643	3,292
Total Loans	690,506	637,325
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	40,401	43,654
Past Due Loans	87	107
Nonaccrual Loans	8	9
Total Loans	40,496	43,770
Financial Asset, 30 to 59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	16,763	18,332
Financial Asset, 30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	5,899	6,421
Financial Asset, 30 to 59 Days Past Due [Member] | Commercial Business [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,055	6,081
Financial Asset, 30 to 59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	2,462	2,283
Financial Asset, 30 to 59 Days Past Due [Member] | Construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	2,238	951
Financial Asset, 30 to 59 Days Past Due [Member] | One-to-Four Family Residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,071	2,516
Financial Asset, 30 to 59 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	38	80
Financial Asset, 60 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	5,407	5,550
Financial Asset, 60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,465	0
Financial Asset, 60 to 89 Days Past Due [Member] | Commercial Business [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,589	2,769
Financial Asset, 60 to 89 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,760	2,346
Financial Asset, 60 to 89 Days Past Due [Member] | Construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Financial Asset, 60 to 89 Days Past Due [Member] | One-to-Four Family Residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	544	408
Financial Asset, 60 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	49	27
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Business [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | One-to-Four Family Residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	$ 0	$ 0