Loans (Analysis of the Aged Loan Portfolio) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Current Loans	$ 9,628,426	$ 9,364,697
Past Due Loans	14,311	28,157
Nonaccrual Loans	33,581	34,806
Total Loans	9,676,318	9,427,660
Commercial Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	4,067,718	4,037,309
Past Due Loans	6,880	17,292
Nonaccrual Loans	7,317	7,712
Total Loans	4,081,915	4,062,313
Commercial Business [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	3,778,955	3,578,905
Past Due Loans	307	5,841
Nonaccrual Loans	13,551	13,222
Total Loans	3,792,813	3,597,968
Agriculture [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	739,599	767,102
Past Due Loans	1,572	911
Nonaccrual Loans	10,629	11,614
Total Loans	751,800	779,627
Construction [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	282,343	268,304
Past Due Loans	0	142
Nonaccrual Loans	191	217
Total Loans	282,534	268,663
One-to-Four Family Residential [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	728,053	677,627
Past Due Loans	5,510	3,942
Nonaccrual Loans	1,751	2,001
Total Loans	735,314	683,570
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Current Loans	31,758	35,450
Past Due Loans	42	29
Nonaccrual Loans	142	40
Total Loans	31,942	35,519
Financial Asset, 30 to 59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	7,538	21,792
Financial Asset, 30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	880	17,292
Financial Asset, 30 to 59 Days Past Due [Member] | Commercial Business [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	109	1,282
Financial Asset, 30 to 59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,060	911
Financial Asset, 30 to 59 Days Past Due [Member] | Construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Financial Asset, 30 to 59 Days Past Due [Member] | One-to-Four Family Residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	5,462	2,283
Financial Asset, 30 to 59 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	27	24
Financial Asset, 60 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	6,773	6,365
Financial Asset, 60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	6,000	0
Financial Asset, 60 to 89 Days Past Due [Member] | Commercial Business [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	198	4,559
Financial Asset, 60 to 89 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	512	0
Financial Asset, 60 to 89 Days Past Due [Member] | Construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	142
Financial Asset, 60 to 89 Days Past Due [Member] | One-to-Four Family Residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	48	1,659
Financial Asset, 60 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due Loans	15	5
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