Loans (Analysis of the Aged Loan Portfolio) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current Loans	$ 9,364,697	$ 8,686,523
Total Past Due	28,157	23,882
Nonaccrual Loans	34,806	33,060
Total Loans, before allowance for credit loss	9,427,660	8,743,465
Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Current Loans	4,037,309	3,935,633
Total Past Due	17,292	6,421
Nonaccrual Loans	7,712	3,799
Total Loans, before allowance for credit loss	4,062,313	3,945,853
Commercial Business
Financing Receivable, Past Due [Line Items]
Current Loans	3,578,905	2,959,826
Total Past Due	5,841	8,850
Nonaccrual Loans	13,222	20,937
Total Loans, before allowance for credit loss	3,597,968	2,989,613
Agriculture
Financing Receivable, Past Due [Line Items]
Current Loans	767,102	755,719
Total Past Due	911	4,629
Nonaccrual Loans	11,614	5,023
Total Loans, before allowance for credit loss	779,627	765,371
Construction
Financing Receivable, Past Due [Line Items]
Current Loans	268,304	360,582
Total Past Due	142	951
Nonaccrual Loans	217	0
Total Loans, before allowance for credit loss	268,663	361,533
Real estate: One-to-four family residential
Financing Receivable, Past Due [Line Items]
Current Loans	677,627	631,109
Total Past Due	3,942	2,924
Nonaccrual Loans	2,001	3,292
Total Loans, before allowance for credit loss	683,570	637,325
Consumer Loan
Financing Receivable, Past Due [Line Items]
Current Loans	35,450	43,654
Total Past Due	29	107
Nonaccrual Loans	40	9
Total Loans, before allowance for credit loss	35,519	43,770
Financial Asset, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	21,792	18,332
Financial Asset, 30 to 59 Days Past Due \| Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	17,292	6,421
Financial Asset, 30 to 59 Days Past Due \| Commercial Business
Financing Receivable, Past Due [Line Items]
Total Past Due	1,282	6,081
Financial Asset, 30 to 59 Days Past Due \| Agriculture
Financing Receivable, Past Due [Line Items]
Total Past Due	911	2,283
Financial Asset, 30 to 59 Days Past Due \| Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	951
Financial Asset, 30 to 59 Days Past Due \| Real estate: One-to-four family residential
Financing Receivable, Past Due [Line Items]
Total Past Due	2,283	2,516
Financial Asset, 30 to 59 Days Past Due \| Consumer Loan
Financing Receivable, Past Due [Line Items]
Total Past Due	24	80
Financial Asset, 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	6,365	5,550
Financial Asset, 60 to 89 Days Past Due \| Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, 60 to 89 Days Past Due \| Commercial Business
Financing Receivable, Past Due [Line Items]
Total Past Due	4,559	2,769
Financial Asset, 60 to 89 Days Past Due \| Agriculture
Financing Receivable, Past Due [Line Items]
Total Past Due	0	2,346
Financial Asset, 60 to 89 Days Past Due \| Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	142	0
Financial Asset, 60 to 89 Days Past Due \| Real estate: One-to-four family residential
Financing Receivable, Past Due [Line Items]
Total Past Due	1,659	408
Financial Asset, 60 to 89 Days Past Due \| Consumer Loan
Financing Receivable, Past Due [Line Items]
Total Past Due	5	27
Financial Asset, Equal to or Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due \| Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due \| Commercial Business
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due \| Agriculture
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due \| Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due \| Real estate: One-to-four family residential
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due \| Consumer Loan
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 0	$ 0