Loans Receivable and the Allowance for Credit Losses - Amortized Cost Basis Based on Delinquency Status (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due
Loans current	$ 3,555,560	$ 3,420,971
Loans held for investment, net of unearned income	3,597,039	3,451,266
90 Days or More Past Due And Accruing	3,238	136
30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	9,080	10,045
60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	5,425	2,283
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	26,974	17,967
Agricultural
Financing Receivable, Recorded Investment, Past Due
Loans current	138,409	137,715
Loans held for investment, net of unearned income	140,837	140,446
90 Days or More Past Due And Accruing	0	0
Agricultural | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	522	975
Agricultural | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	351	0
Agricultural | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,555	1,756
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due
Loans current	1,078,603	828,842
Loans held for investment, net of unearned income	1,084,527	835,236
90 Days or More Past Due And Accruing	35	0
Commercial and industrial | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	245	846
Commercial and industrial | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,134	270
Commercial and industrial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	4,545	5,278
Commercial real estate
Financing Receivable, Recorded Investment, Past Due
Loans current	1,741,810	1,799,793
Loans held for investment, net of unearned income	1,764,739	1,814,859
90 Days or More Past Due And Accruing	0	0
Commercial real estate | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	5,483	4,977
Commercial real estate | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,928	968
Commercial real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	15,518	9,121
Commercial real estate | Construction & development
Financing Receivable, Recorded Investment, Past Due
Loans current	194,965	294,995
Loans held for investment, net of unearned income	199,950	298,077
90 Days or More Past Due And Accruing	0	0
Commercial real estate | Construction & development | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	3,663	2,256
Commercial real estate | Construction & development | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	707	621
Commercial real estate | Construction & development | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	615	205
Commercial real estate | Farmland
Financing Receivable, Recorded Investment, Past Due
Loans current	151,530	175,281
Loans held for investment, net of unearned income	161,897	181,885
90 Days or More Past Due And Accruing	0	0
Commercial real estate | Farmland | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	388	362
Commercial real estate | Farmland | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,221	0
Commercial real estate | Farmland | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	8,758	6,242
Commercial real estate | Multifamily
Financing Receivable, Recorded Investment, Past Due
Loans current	247,403	227,013
Loans held for investment, net of unearned income	247,403	227,407
90 Days or More Past Due And Accruing	0	0
Commercial real estate | Multifamily | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	0	394
Commercial real estate | Multifamily | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	0	0
Commercial real estate | Multifamily | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	0	0
Commercial real estate | Commercial real estate-other
Financing Receivable, Recorded Investment, Past Due
Loans current	1,147,912	1,102,504
Loans held for investment, net of unearned income	1,155,489	1,107,490
90 Days or More Past Due And Accruing	0	0
Commercial real estate | Commercial real estate-other | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,432	1,965
Commercial real estate | Commercial real estate-other | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	0	347
Commercial real estate | Commercial real estate-other | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	6,145	2,674
Residential real estate
Financing Receivable, Recorded Investment, Past Due
Loans current	522,970	572,063
Loans held for investment, net of unearned income	532,914	577,799
90 Days or More Past Due And Accruing	3,197	124
Residential real estate | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	2,694	3,097
Residential real estate | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,999	965
Residential real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	5,251	1,674
Residential real estate | One- to four- family first liens
Financing Receivable, Recorded Investment, Past Due
Loans current	368,201	402,471
Loans held for investment, net of unearned income	377,100	407,418
90 Days or More Past Due And Accruing	3,197	99
Residential real estate | One- to four- family first liens | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	2,142	2,579
Residential real estate | One- to four- family first liens | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,740	857
Residential real estate | One- to four- family first liens | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	5,017	1,511
Residential real estate | One- to four- family junior liens
Financing Receivable, Recorded Investment, Past Due
Loans current	154,769	169,592
Loans held for investment, net of unearned income	155,814	170,381
90 Days or More Past Due And Accruing	0	25
Residential real estate | One- to four- family junior liens | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	552	518
Residential real estate | One- to four- family junior liens | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	259	108
Residential real estate | One- to four- family junior liens | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	234	163
Consumer
Financing Receivable, Recorded Investment, Past Due
Loans current	73,768	82,558
Loans held for investment, net of unearned income	74,022	82,926
90 Days or More Past Due And Accruing	6	12
Consumer | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	136	150
Consumer | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	13	80
Consumer | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	$ 105	$ 138