Loans Receivable and the Allowance for Credit Losses - Primary Type of Collateral (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	$ 3,541,395			$ 3,422,187
Allowance for credit losses on loans	55,644	$ 51,187	$ 29,079	29,079	$ 28,691	$ 29,652	$ 29,307
Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	25,272
Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	6,085
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	3,909
Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	35,266
Allowance for credit losses on loans	3,179
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses on loans	1,408	1,146	3,748	3,748	3,720	3,691	3,637
Agricultural | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	124
Agricultural | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	2,011
Agricultural | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	612
Agricultural | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	2,747
Allowance for credit losses on loans	490
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses on loans	18,709	19,309	8,394	8,394	7,633	7,594	7,478
Commercial and industrial | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	1,230
Commercial and industrial | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	4,066
Commercial and industrial | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	3,297
Commercial and industrial | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	8,593
Allowance for credit losses on loans	1,774
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses on loans	28,221	23,138	13,804	13,804	13,655	14,798	15,635
Commercial real estate | Construction & development | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	700
Commercial real estate | Construction & development | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial real estate | Construction & development | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial real estate | Construction & development | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	700
Allowance for credit losses on loans	0
Commercial real estate | Farmland | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	13,646
Commercial real estate | Farmland | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial real estate | Farmland | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial real estate | Farmland | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	13,646
Allowance for credit losses on loans	283
Commercial real estate | Multifamily | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial real estate | Multifamily | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial real estate | Multifamily | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial real estate | Multifamily | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Allowance for credit losses on loans	0
Commercial real estate | Commercial real estate-other | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	9,368
Commercial real estate | Commercial real estate-other | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial real estate | Commercial real estate-other | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial real estate | Commercial real estate-other | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	9,368
Allowance for credit losses on loans	632
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses on loans	6,074	6,425	2,685	2,685	3,377	3,272	2,349
Residential real estate | One- to four- family first liens | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	204
Residential real estate | One- to four- family first liens | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Residential real estate | One- to four- family first liens | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Residential real estate | One- to four- family first liens | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	204
Allowance for credit losses on loans	0
Residential real estate | One- to four- family junior liens | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Residential real estate | One- to four- family junior liens | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Residential real estate | One- to four- family junior liens | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Residential real estate | One- to four- family junior liens | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Allowance for credit losses on loans	0
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses on loans	1,232	$ 1,169	$ 448	$ 448	$ 306	$ 297	$ 208
Consumer | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Consumer | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	8
Consumer | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Consumer | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	8
Allowance for credit losses on loans	$ 0