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Allowance for Credit Losses and Unfunded Commitments and Letters of Credit (Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020	Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Less: ACL	$ 149,140				$ 83,968				$ 149,140	$ 83,968	$ 83,369		$ 75,646
Financing Receivable, Allowance for Credit Loss, Writeoff									21,092	15,215	18,555
Financing Receivable, Allowance for Credit Loss, Recovery									6,932	12,321	11,509
Provision (recapture) for credit losses	(4,700)	$ 7,400	$ 33,500	$ 41,500	1,614	$ 299	$ 218	$ 1,362	77,700	3,493	14,769
Commercial Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Less: ACL	68,934				20,340				68,934	20,340	14,864		16,260
Financing Receivable, Allowance for Credit Loss, Writeoff									1,419	2,160	3,840
Financing Receivable, Allowance for Credit Loss, Recovery									131	3,377	3,186
Provision (recapture) for credit losses									42,349	4,259	(742)
Commercial Business
Allowance for Loan and Lease Losses [Roll Forward]
Less: ACL	45,250				30,292				45,250	30,292	34,658		25,101
Financing Receivable, Allowance for Credit Loss, Writeoff									12,396	11,290	7,437
Financing Receivable, Allowance for Credit Loss, Recovery									3,438	3,066	2,829
Provision (recapture) for credit losses									23,154	3,858	14,165
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Less: ACL	9,052				15,835				9,052	15,835	9,589		9,662
Financing Receivable, Allowance for Credit Loss, Writeoff									6,427	245	5,507
Financing Receivable, Allowance for Credit Loss, Recovery									172	299	1,104
Provision (recapture) for credit losses									8,797	6,192	4,330
Construction
Allowance for Loan and Lease Losses [Roll Forward]
Less: ACL	7,636				8,571				7,636	8,571	14,395		15,092
Financing Receivable, Allowance for Credit Loss, Writeoff									0	242	124
Financing Receivable, Allowance for Credit Loss, Recovery									709	3,641	1,686
Provision (recapture) for credit losses									106	(9,223)	(2,259)
Real estate: One-to-four family residential
Allowance for Loan and Lease Losses [Roll Forward]
Less: ACL	16,875				7,435				16,875	7,435	8,024		8,904
Financing Receivable, Allowance for Credit Loss, Writeoff									84	1,196	1,451
Financing Receivable, Allowance for Credit Loss, Recovery									2,083	1,773	2,516
Provision (recapture) for credit losses									3,204	(1,166)	(1,945)
Consumer Loan
Allowance for Loan and Lease Losses [Roll Forward]
Less: ACL	1,393				883				1,393	883	786		627
Financing Receivable, Allowance for Credit Loss, Writeoff									766	82	196
Financing Receivable, Allowance for Credit Loss, Recovery									399	165	188
Provision (recapture) for credit losses									99	14	167
Unallocated Financing Receivables [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Less: ACL	$ 0				$ 612				0	612	1,053		$ 0
Financing Receivable, Allowance for Credit Loss, Writeoff									0	0	0
Financing Receivable, Allowance for Credit Loss, Recovery									0	0	0
Provision (recapture) for credit losses									$ (9)	$ (441)	$ 1,053
Accounting Standards Update 2016-13
Allowance for Loan and Lease Losses [Roll Forward]
Change in allowance for credit loss due to adoption of new ASU												$ 1,632
Accounting Standards Update 2016-13 | Commercial Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Change in allowance for credit loss due to adoption of new ASU												7,533
Accounting Standards Update 2016-13 | Commercial Business
Allowance for Loan and Lease Losses [Roll Forward]
Change in allowance for credit loss due to adoption of new ASU												762
Accounting Standards Update 2016-13 | Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Change in allowance for credit loss due to adoption of new ASU												(9,325)
Accounting Standards Update 2016-13 | Construction
Allowance for Loan and Lease Losses [Roll Forward]
Change in allowance for credit loss due to adoption of new ASU												(1,750)
Accounting Standards Update 2016-13 | Real estate: One-to-four family residential
Allowance for Loan and Lease Losses [Roll Forward]
Change in allowance for credit loss due to adoption of new ASU												4,237
Accounting Standards Update 2016-13 | Consumer Loan
Allowance for Loan and Lease Losses [Roll Forward]
Change in allowance for credit loss due to adoption of new ASU												778
Accounting Standards Update 2016-13 | Unallocated Financing Receivables [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Change in allowance for credit loss due to adoption of new ASU												$ (603)

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Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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