Allowance for Credit Losses and Allowance for Unfunded Commitments and Letters of Credit (Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	$ 122,074	$ 83,274	$ 85,600	$ 83,968	$ 83,369
Financing Receivable, Allowance for Credit Loss, Writeoff	(6,789)	(2,988)
Financing Receivable, Allowance for Credit Loss, Recovery	1,763	1,531
Provision for credit losses	41,500	1,362
Allowance on Individually Measured Loans	2,848	2,911
Allowance on Collectively Measured Loans	119,226	80,363
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	37,122	16,359	27,873	20,340	14,766
Financing Receivable, Allowance for Credit Loss, Writeoff	(101)	(678)
Financing Receivable, Allowance for Credit Loss, Recovery	14	514
Provision for credit losses	9,336	1,757
Allowance on Individually Measured Loans	0	53
Allowance on Collectively Measured Loans	37,122	16,306
Commercial Business [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	45,570	34,545	31,054	30,292	34,658
Financing Receivable, Allowance for Credit Loss, Writeoff	(1,684)	(1,506)
Financing Receivable, Allowance for Credit Loss, Recovery	860	527
Provision for credit losses	15,340	866
Allowance on Individually Measured Loans	1,722	2,826
Allowance on Collectively Measured Loans	43,848	31,719
Agriculture [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	11,085	10,690	6,510	15,835	9,589
Financing Receivable, Allowance for Credit Loss, Writeoff	(4,726)	(78)
Financing Receivable, Allowance for Credit Loss, Recovery	41	58
Provision for credit losses	9,260	1,121
Allowance on Individually Measured Loans	1,126	0
Allowance on Collectively Measured Loans	9,959	10,690
Construction [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	8,845	11,965	6,821	8,571	14,395
Financing Receivable, Allowance for Credit Loss, Writeoff	0	(195)
Financing Receivable, Allowance for Credit Loss, Recovery	442	83
Provision for credit losses	1,582	(2,318)
Allowance on Individually Measured Loans	0	0
Allowance on Collectively Measured Loans	8,845	11,965
One-to-Four Family Residential [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	17,659	8,311	11,672	7,435	8,024
Financing Receivable, Allowance for Credit Loss, Writeoff	(10)	(481)
Financing Receivable, Allowance for Credit Loss, Recovery	282	334
Provision for credit losses	5,715	434
Allowance on Individually Measured Loans	0	31
Allowance on Collectively Measured Loans	17,659	8,280
Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	1,644	823	1,661	883	787
Financing Receivable, Allowance for Credit Loss, Writeoff	(268)	(50)
Financing Receivable, Allowance for Credit Loss, Recovery	124	15
Provision for credit losses	127	71
Allowance on Individually Measured Loans	0	1
Allowance on Collectively Measured Loans	1,644	822
Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	149	581	9	$ 612	$ 1,150
Financing Receivable, Allowance for Credit Loss, Writeoff	0	0
Financing Receivable, Allowance for Credit Loss, Recovery	0	0
Provision for credit losses	140	(569)
Allowance on Individually Measured Loans	0	0
Allowance on Collectively Measured Loans	$ 149	$ 581
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)