Loans Receivable and the Allowance for Credit Losses - Amortized Cost Basis Based on Delinquency Status (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due
Loans current	$ 3,382,313	$ 3,420,971
Loans held for investment, net of unearned income	3,425,762	3,451,266
90 Days or More Past Due And Accruing	303	136
30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	15,269	10,045
60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	5,185	2,283
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	22,995	17,967
Agricultural
Financing Receivable, Recorded Investment, Past Due
Loans current	137,398	137,715
Loans held for investment, net of unearned income	145,435	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	6,416	975
Agricultural | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	416	0
Agricultural | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,205	1,756
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due
Loans current	858,489	828,842
Loans held for investment, net of unearned income	864,702	835,236
90 Days or More Past Due And Accruing	62	0
Commercial and industrial | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,999	846
Commercial and industrial | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	436	270
Commercial and industrial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	3,778	5,278
Commercial real estate
Financing Receivable, Recorded Investment, Past Due
Loans current	1,757,239	1,799,793
Loans held for investment, net of unearned income	1,780,446	1,814,859
90 Days or More Past Due And Accruing	174	0
Commercial real estate | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	4,044	4,977
Commercial real estate | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	3,158	968
Commercial real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	16,005	9,121
Commercial real estate | Construction & development
Financing Receivable, Recorded Investment, Past Due
Loans current	281,109	294,995
Loans held for investment, net of unearned income	282,921	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	431	2,256
Commercial real estate | Construction & development | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	649	621
Commercial real estate | Construction & development | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	732	205
Commercial real estate | Farmland
Financing Receivable, Recorded Investment, Past Due
Loans current	157,371	175,281
Loans held for investment, net of unearned income	168,777	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	1,760	362
Commercial real estate | Farmland | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	2,062	0
Commercial real estate | Farmland | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	7,584	6,242
Commercial real estate | Multifamily
Financing Receivable, Recorded Investment, Past Due
Loans current	216,242	227,013
Loans held for investment, net of unearned income	217,108	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	866	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	1,102,517	1,102,504
Loans held for investment, net of unearned income	1,111,640	1,107,490
90 Days or More Past Due And Accruing	174	0
Commercial real estate | Commercial real estate-other | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	987	1,965
Commercial real estate | Commercial real estate-other | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	447	347
Commercial real estate | Commercial real estate-other | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	7,689	2,674
Residential real estate
Financing Receivable, Recorded Investment, Past Due
Loans current	548,659	572,063
Loans held for investment, net of unearned income	554,290	577,799
90 Days or More Past Due And Accruing	44	124
Residential real estate | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	2,670	3,097
Residential real estate | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,129	965
Residential real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	1,832	1,674
Residential real estate | One- to four- family first liens
Financing Receivable, Recorded Investment, Past Due
Loans current	384,069	402,471
Loans held for investment, net of unearned income	389,055	407,418
90 Days or More Past Due And Accruing	44	99
Residential real estate | One- to four- family first liens | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	2,339	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	993	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	1,654	1,511
Residential real estate | One- to four- family junior liens
Financing Receivable, Recorded Investment, Past Due
Loans current	164,590	169,592
Loans held for investment, net of unearned income	165,235	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	331	518
Residential real estate | One- to four- family junior liens | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	136	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	178	163
Consumer
Financing Receivable, Recorded Investment, Past Due
Loans current	80,528	82,558
Loans held for investment, net of unearned income	80,889	82,926
90 Days or More Past Due And Accruing	23	12
Consumer | 30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	140	150
Consumer | 60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	46	80
Consumer | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Loans past due	$ 175	$ 138