Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 4,197	$ 4,034	$ 3,806	$ 3,129
Impaired Financing Receivable, Interest Income, Accrual Method	21	1	23	1
Related allowance	25		25		$ 131
Recorded investment	7,080		7,080		3,292
Unpaid principal balance	7,572		7,572		4,877
Acquired Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		131
Recorded investment	0		0		131
Unpaid principal balance	0		0		131
Acquired Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,630	299	1,035	355
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	4,718		4,718		73
Unpaid principal balance	4,718		4,718		73
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Acquired Loans [Member] | Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,969	1,962	1,975	1,366
Impaired Financing Receivable, Interest Income, Accrual Method	4	1	5	1
Recorded investment	1,939		1,939		2,031
Unpaid principal balance	2,391		2,391		2,604
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	423	1,692	620	1,359
Impaired Financing Receivable, Interest Income, Accrual Method	17	0	18	0
Recorded investment	251		251		1,042
Unpaid principal balance	276		276		2,052
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	14	16	14	17
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	13		13		15
Unpaid principal balance	15		15		17
Acquired Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	4,036	3,969	3,644	3,097
Impaired Financing Receivable, Interest Income, Accrual Method	21	1	23	1
Recorded investment	6,921		6,921		3,161
Unpaid principal balance	7,400		7,400		4,746
Acquired Loans [Member] | Real Estate Construction With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Acquired Loans [Member] | Residential Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	65	0	32
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	161	0	162	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	25		25		0
Recorded investment	159		159		0
Unpaid principal balance	172		172		0
Acquired Loans [Member] | Loans To Individuals With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Acquired Loans [Member] | With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	161	65	162	32
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	25		25		131
Recorded investment	159		159		131
Unpaid principal balance	172		172		131
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	27,815	50,748	26,294	47,764
Impaired Financing Receivable, Interest Income, Accrual Method	214	633	345	807
Related allowance	2,505		2,505		1,500
Recorded investment	28,474		28,474		28,735
Unpaid principal balance	37,371		37,371		38,787
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,957	22,812	3,784	19,694
Impaired Financing Receivable, Interest Income, Accrual Method	16	23	30	52
Related allowance	1,330		1,330		797
Recorded investment	4,195		4,195		3,042
Unpaid principal balance	5,886		5,886		3,181
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	2,268	6,793	2,216	8,034
Impaired Financing Receivable, Interest Income, Accrual Method	5	481	7	528
Recorded investment	2,767		2,767		8,735
Unpaid principal balance	7,828		7,828		16,442
Originated Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Originated Loans [Member] | Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	10,242	10,334	10,577	10,316
Impaired Financing Receivable, Interest Income, Accrual Method	115	64	186	125
Recorded investment	10,092		10,092		10,726
Unpaid principal balance	11,735		11,735		12,571
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,034	6,086	3,256	6,076
Impaired Financing Receivable, Interest Income, Accrual Method	65	23	101	54
Recorded investment	3,042		3,042		3,599
Unpaid principal balance	3,265		3,265		3,812
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	357	292	340	322
Impaired Financing Receivable, Interest Income, Accrual Method	5	3	7	4
Recorded investment	361		361		281
Unpaid principal balance	550		550		408
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	15,901	23,505	16,389	24,748
Impaired Financing Receivable, Interest Income, Accrual Method	190	571	301	711
Recorded investment	16,262		16,262		23,341
Unpaid principal balance	23,378		23,378		33,233
Originated Loans [Member] | Real Estate Construction With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Originated Loans [Member] | Residential Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	653	880	571	845
Impaired Financing Receivable, Interest Income, Accrual Method	7	5	12	9
Related allowance	87		87		107
Recorded investment	761		761		486
Unpaid principal balance	760		760		495
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	7,304	3,551	5,550	2,477
Impaired Financing Receivable, Interest Income, Accrual Method	1	34	2	35
Related allowance	1,088		1,088		596
Recorded investment	7,256		7,256		1,866
Unpaid principal balance	7,347		7,347		1,878
Originated Loans [Member] | Loans To Individuals With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Originated Loans [Member] | With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	11,914	27,243	9,905	23,016
Impaired Financing Receivable, Interest Income, Accrual Method	24	$ 62	44	$ 96
Related allowance	2,505		2,505		1,500
Recorded investment	12,212		12,212		5,394
Unpaid principal balance	$ 13,993		$ 13,993		$ 5,554