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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 7,481	$ 4,585	$ 5,031	$ 3,614
Impaired Financing Receivable, Interest Income, Accrual Method	1	12	24	13
Related allowance	19		19		$ 131
Recorded investment	8,289		8,289		3,292
Unpaid principal balance	10,153		10,153		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	4,697	73	2,255	261
Impaired Financing Receivable, Interest Income, Accrual Method	0	10	0	10
Recorded investment	4,693		4,693		73
Unpaid principal balance	4,717		4,717		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	1,950	2,541	1,966	1,779
Impaired Financing Receivable, Interest Income, Accrual Method	1	2	6	3
Recorded investment	1,971		1,971		2,031
Unpaid principal balance	2,413		2,413		2,604
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	666	1,955	636	1,558
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	18	0
Recorded investment	1,459		1,459		1,042
Unpaid principal balance	2,843		2,843		2,052
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	13	16	14	16
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	7,326	4,585	4,871	3,614
Impaired Financing Receivable, Interest Income, Accrual Method	1	12	24	13
Recorded investment	8,136		8,136		3,161
Unpaid principal balance	9,988		9,988		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	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	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	155	0	160	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	19		19		0
Recorded investment	153		153		0
Unpaid principal balance	165		165		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	155	0	160	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	19		19		131
Recorded investment	153		153		131
Unpaid principal balance	165		165		131
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	27,987	38,659	26,858	44,729
Impaired Financing Receivable, Interest Income, Accrual Method	131	135	474	949
Related allowance	1,527		1,527		1,500
Recorded investment	27,036		27,036		28,735
Unpaid principal balance	36,530		36,530		38,787
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	4,677	10,298	4,064	5,979
Impaired Financing Receivable, Interest Income, Accrual Method	8	4	36	16
Related allowance	1,054		1,054		797
Recorded investment	4,644		4,644		3,042
Unpaid principal balance	6,386		6,386		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	1,902	5,697	2,129	17,838
Impaired Financing Receivable, Interest Income, Accrual Method	3	8	10	576
Recorded investment	1,638		1,638		8,735
Unpaid principal balance	7,014		7,014		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	10,254	10,627	10,751	10,639
Impaired Financing Receivable, Interest Income, Accrual Method	86	59	280	191
Recorded investment	9,760		9,760		10,726
Unpaid principal balance	11,516		11,516		12,571
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,582	6,810	3,854	7,632
Impaired Financing Receivable, Interest Income, Accrual Method	28	59	129	146
Recorded investment	3,274		3,274		3,599
Unpaid principal balance	3,502		3,502		3,812
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	389	320	356	322
Impaired Financing Receivable, Interest Income, Accrual Method	4	2	11	6
Recorded investment	392		392		281
Unpaid principal balance	593		593		408
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	16,127	23,454	17,090	36,431
Impaired Financing Receivable, Interest Income, Accrual Method	121	128	430	919
Recorded investment	15,064		15,064		23,341
Unpaid principal balance	22,625		22,625		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	740	565	347	532
Impaired Financing Receivable, Interest Income, Accrual Method	1	2	6	11
Related allowance	4		4		107
Recorded investment	920		920		486
Unpaid principal balance	976		976		495
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	6,443	4,342	5,357	1,787
Impaired Financing Receivable, Interest Income, Accrual Method	1	1	2	3
Related allowance	469		469		596
Recorded investment	6,408		6,408		1,866
Unpaid principal balance	6,543		6,543		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	11,860	15,205	9,768	8,298
Impaired Financing Receivable, Interest Income, Accrual Method	10	$ 7	44	$ 30
Related allowance	1,527		1,527		1,500
Recorded investment	11,972		11,972		5,394
Unpaid principal balance	$ 13,905		$ 13,905		$ 5,554