Loans Receivable and the Allowance for Credit Losses - Composition of Loans by Lending Classification (Details) - USD ($)	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, gross	$ 3,597,039,000			$ 3,451,266,000
Allowance for credit losses	(55,644,000)	$ (51,187,000)	$ (29,079,000)	(29,079,000)	$ (28,691,000)	$ (29,652,000)	$ (29,307,000)
Total loans held for investment, net	3,541,395,000			3,422,187,000
Loans pledged for Federal Home Loan Bank debt	889,300,000			945,900,000
Loans and leases receivable, impaired, commitment to lend	0
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	140,837,000			140,446,000
Allowance for credit losses	(1,408,000)	(1,146,000)	(3,748,000)	(3,748,000)	(3,720,000)	(3,691,000)	(3,637,000)
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	1,084,527,000			835,236,000
Allowance for credit losses	(18,709,000)	(19,309,000)	(8,394,000)	(8,394,000)	(7,633,000)	(7,594,000)	(7,478,000)
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	1,764,739,000			1,814,859,000
Allowance for credit losses	(28,221,000)	(23,138,000)	(13,804,000)	(13,804,000)	(13,655,000)	(14,798,000)	(15,635,000)
Commercial real estate | Construction & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	199,950,000			298,077,000
Commercial real estate | Farmland
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	161,897,000			181,885,000
Commercial real estate | Multifamily
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	247,403,000			227,407,000
Commercial real estate | Commercial real estate-other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	1,155,489,000			1,107,490,000
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	532,914,000			577,799,000
Allowance for credit losses	(6,074,000)	(6,425,000)	(2,685,000)	(2,685,000)	(3,377,000)	(3,272,000)	(2,349,000)
Residential real estate | One- to four- family first liens
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	377,100,000			407,418,000
Residential real estate | One- to four- family junior liens
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	155,814,000			170,381,000
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	74,022,000			82,926,000
Allowance for credit losses	$ (1,232,000)	$ (1,169,000)	$ (448,000)	$ (448,000)	$ (306,000)	$ (297,000)	$ (208,000)