Loans Receivable and the Allowance for Credit Losses - Primary Type of Collateral (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	$ 3,374,575		$ 3,422,187
Allowance for credit losses on loans	51,187	$ 29,079	29,079	$ 29,652	$ 29,307
Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	26,715
Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	7,432
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	4,383
Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	38,530
Allowance for credit losses on loans	3,925
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	145,435
Allowance for credit losses on loans	1,146	3,748	3,748	3,691	3,637
Agricultural | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	1,181
Agricultural | Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Agricultural | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	1,708
Agricultural | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	367
Agricultural | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	3,256
Allowance for credit losses on loans	251
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	864,702
Allowance for credit losses on loans	19,309	8,394	8,394	7,594	7,478
Commercial and industrial | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	1,060
Commercial and industrial | Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial and industrial | Equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	5,703
Commercial and industrial | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	4,016
Commercial and industrial | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	10,779
Allowance for credit losses on loans	1,920
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses on loans	23,138	13,804	13,804	14,798	15,635
Commercial real estate | Construction & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	282,921
Commercial real estate | Construction & development | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	700
Commercial real estate | Construction & development | Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
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
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	168,777
Commercial real estate | Farmland | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	13,195
Commercial real estate | Farmland | Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
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	13,195
Allowance for credit losses on loans	657
Commercial real estate | Multifamily
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	217,108
Commercial real estate | Multifamily | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Commercial real estate | Multifamily | Accounts Receivable
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
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	1,111,640
Commercial real estate | Commercial real estate-other | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	9,869
Commercial real estate | Commercial real estate-other | Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
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	9,869
Allowance for credit losses on loans	994
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses on loans	6,425	2,685	2,685	3,272	2,349
Residential real estate | One- to four- family first liens
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	389,055
Residential real estate | One- to four- family first liens | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	710
Residential real estate | One- to four- family first liens | Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
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	710
Allowance for credit losses on loans	103
Residential real estate | One- to four- family junior liens
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	165,235
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 | Accounts Receivable
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]
Loans receivable, net	80,889
Allowance for credit losses on loans	1,169	$ 448	$ 448	$ 297	$ 208
Consumer | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	0
Consumer | Accounts Receivable
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	21
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	21
Allowance for credit losses on loans	$ 0