Allowance for Credit Losses and Allowance for Unfunded Commitments and Letters of Credit (Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	$ 156,968	$ 82,660	$ 156,968	$ 82,660	$ 151,546	$ 85,600	$ 83,968	$ 80,517	$ 83,369
Financing Receivable, Allowance for Credit Loss, Writeoff	(4,582)	(3,372)	(17,011)	(11,647)
Financing Receivable, Allowance for Credit Loss, Recovery	2,604	5,216	5,979	9,059
Provision for credit losses	7,400	299	82,400	1,879
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	61,324	17,922	61,324	17,922	50,233	27,873	20,340	15,648	14,766
Financing Receivable, Allowance for Credit Loss, Writeoff	0	(466)	(101)	(1,708)
Financing Receivable, Allowance for Credit Loss, Recovery	65	1,731	92	2,801
Provision for credit losses	11,026	1,009	33,460	2,063
Commercial Business [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	50,572	30,879	50,572	30,879	53,186	31,054	30,292	29,700	34,658
Financing Receivable, Allowance for Credit Loss, Writeoff	(3,164)	(2,623)	(10,290)	(8,445)
Financing Receivable, Allowance for Credit Loss, Recovery	1,124	349	2,795	1,368
Provision for credit losses	(574)	3,453	27,013	3,298
Agriculture [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	12,461	13,899	12,461	13,899	14,868	6,510	15,835	12,671	9,589
Financing Receivable, Allowance for Credit Loss, Writeoff	(1,269)	(55)	(5,995)	(194)
Financing Receivable, Allowance for Credit Loss, Recovery	27	67	69	189
Provision for credit losses	(1,165)	1,216	11,877	4,315
Construction [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	8,963	11,852	8,963	11,852	7,953	6,821	8,571	12,457	14,395
Financing Receivable, Allowance for Credit Loss, Writeoff	0	(17)	0	(232)
Financing Receivable, Allowance for Credit Loss, Recovery	11	2,555	688	3,329
Provision for credit losses	999	(3,143)	1,454	(5,640)
One-to-Four Family Residential [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	22,107	7,458	22,107	7,458	23,711	11,672	7,435	7,619	8,024
Financing Receivable, Allowance for Credit Loss, Writeoff	(16)	(202)	(26)	(1,004)
Financing Receivable, Allowance for Credit Loss, Recovery	1,301	440	2,005	1,224
Provision for credit losses	(2,889)	(399)	8,456	(786)
Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	1,541	579	1,541	579	$ 1,595	1,661	883	715	787
Financing Receivable, Allowance for Credit Loss, Writeoff	(133)	(9)	(599)	(64)
Financing Receivable, Allowance for Credit Loss, Recovery	76	74	330	148
Provision for credit losses	3	(201)	149	(292)
Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	$ 0	71	0	71		9	$ 612	$ 1,707	$ 1,150
Financing Receivable, Allowance for Credit Loss, Writeoff		0	0	0
Financing Receivable, Allowance for Credit Loss, Recovery		0	0	0
Provision for credit losses		$ (1,636)	$ (9)	$ (1,079)
Accounting Standards Update 2016-13 [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Change in allowance for credit losses due to adoption of new ASU						1,632
Accounting Standards Update 2016-13 [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Change in allowance for credit losses due to adoption of new ASU						7,533
Accounting Standards Update 2016-13 [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Change in allowance for credit losses due to adoption of new ASU						762
Accounting Standards Update 2016-13 [Member] | Agriculture [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Change in allowance for credit losses due to adoption of new ASU						(9,325)
Accounting Standards Update 2016-13 [Member] | Construction [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Change in allowance for credit losses due to adoption of new ASU						(1,750)
Accounting Standards Update 2016-13 [Member] | One-to-Four Family Residential [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Change in allowance for credit losses due to adoption of new ASU						4,237
Accounting Standards Update 2016-13 [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Change in allowance for credit losses due to adoption of new ASU						778
Accounting Standards Update 2016-13 [Member] | Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Change in allowance for credit losses due to adoption of new ASU						$ (603)