Loans and Allowance for Loan Losses - Transactions in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Allowance for loan losses:
Beginning balance	$ 27,174	$ 23,909	$ 23,909
Provision	1,187	2,113	4,298
Charge-offs	(570)	(583)	(1,663)
Recoveries	479	299	630
Ending balance	28,270	25,738	27,174
Ending balance individually evaluated for impairment	1,149	1,303	1,164
Ending balance collectively evaluated for impairment	27,121	24,435	26,010
Loans:
Ending balance	2,027,822	1,913,614	2,050,199
Ending balance individually evaluated for impairment	6,869	7,745	5,346
Ending balance collectively evaluated for impairment	2,020,953	1,905,869	2,044,853
Mortgage loans on real estate:
Loans:
Ending balance	1,877,911		1,911,886
Mortgage loans on real estate: | Residential 1-4 family
Allowance for loan losses:
Beginning balance	6,297	5,156	5,156
Provision	238	608	1,568
Charge-offs	0	(28)	(492)
Recoveries	16	15	65
Ending balance	6,551	5,751	6,297
Ending balance individually evaluated for impairment	828	919	852
Ending balance collectively evaluated for impairment	5,723	4,832	5,445
Loans:
Ending balance	464,115	420,332	460,692
Ending balance individually evaluated for impairment	2,672	4,110	2,829
Ending balance collectively evaluated for impairment	461,443	416,222	457,863
Mortgage loans on real estate: | Multifamily
Allowance for loan losses:
Beginning balance	1,481	1,011	1,011
Provision	(89)	44	470
Charge-offs	0	0	0
Recoveries	0	0	0
Ending balance	1,392	1,055	1,481
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	1,392	1,055	1,481
Loans:
Ending balance	121,044	100,491	134,613
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	121,044	100,491	134,613
Mortgage loans on real estate: | Commercial
Allowance for loan losses:
Beginning balance	9,753	9,267	9,267
Provision	987	232	436
Charge-offs	0	0	0
Recoveries	0	0	50
Ending balance	10,740	9,499	9,753
Ending balance individually evaluated for impairment	321	384	312
Ending balance collectively evaluated for impairment	10,419	9,115	9,441
Loans:
Ending balance	743,322	701,988	701,055
Ending balance individually evaluated for impairment	4,197	2,481	1,831
Ending balance collectively evaluated for impairment	739,125	699,507	699,224
Mortgage loans on real estate: | Construction
Allowance for loan losses:
Beginning balance	7,084	6,094	6,094
Provision	(927)	1,008	921
Charge-offs	0	(2)	(19)
Recoveries	288	42	88
Ending balance	6,445	7,142	7,084
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	6,445	7,142	7,084
Loans:
Ending balance	450,199	477,439	518,245
Ending balance individually evaluated for impairment	0	844	686
Ending balance collectively evaluated for impairment	450,199	476,595	517,559
Mortgage loans on real estate: | Farmland
Allowance for loan losses:
Beginning balance	221	487	487
Provision	(9)	(220)	(266)
Charge-offs	0	0	0
Recoveries	0	0	0
Ending balance	212	267	221
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	212	267	221
Loans:
Ending balance	21,843	25,569	24,071
Ending balance individually evaluated for impairment	0	310	0
Ending balance collectively evaluated for impairment	21,843	25,259	24,071
Mortgage loans on real estate: | Second mortgages
Allowance for loan losses:
Beginning balance	118	94	94
Provision	14	6	24
Charge-offs	0	0	0
Recoveries	0	0	0
Ending balance	132	100	118
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	132	100	118
Loans:
Ending balance	11,859	9,936	11,197
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	11,859	9,936	11,197
Mortgage loans on real estate: | Equity lines of credit
Allowance for loan losses:
Beginning balance	731	723	723
Provision	74	64	7
Charge-offs	0	0	0
Recoveries	0	1	1
Ending balance	805	788	731
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	805	788	731
Loans:
Ending balance	65,529	70,469	62,013
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	65,529	70,469	62,013
Commercial loans
Allowance for loan losses:
Beginning balance	622	401	401
Provision	281	1	218
Charge-offs	(15)	0	0
Recoveries	0	3	3
Ending balance	888	405	622
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	888	405	622
Loans:
Ending balance	83,969	51,098	78,245
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	83,969	51,098	78,245
Agricultural, Installment and Other
Allowance for loan losses:
Beginning balance	867	676	676
Provision	618	370	920
Charge-offs	(555)	(553)	(1,152)
Recoveries	175	238	423
Ending balance	1,105	731	867
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	1,105	731	867
Loans:
Ending balance	65,942	56,292	60,068
Ending balance individually evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	$ 65,942	$ 56,292	$ 60,068