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
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 3,416	$ 2,224
Impaired Financing Receivable, Interest Income, Accrual Method	2	1
Related allowance	0		$ 131
Recorded investment	2,998		3,292
Unpaid principal balance	4,447		4,877
Acquired Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		131
Recorded investment	0		131
Unpaid principal balance	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	439	411
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	100		73
Unpaid principal balance	100		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
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,981	678
Impaired Financing Receivable, Interest Income, Accrual Method	1	1
Recorded investment	1,937		2,031
Unpaid principal balance	2,369		2,604
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	982	907
Impaired Financing Receivable, Interest Income, Accrual Method	1	0
Recorded investment	947		1,042
Unpaid principal balance	1,962		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	17
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	14		15
Unpaid principal balance	16		17
Acquired Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,416	2,013
Impaired Financing Receivable, Interest Income, Accrual Method	2	1
Recorded investment	2,998		3,161
Unpaid principal balance	4,447		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
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Residential Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	93
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	118
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Loans To Individuals With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	211
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		131
Recorded investment	0		131
Unpaid principal balance	0		131
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	24,773	44,781
Impaired Financing Receivable, Interest Income, Accrual Method	131	173
Related allowance	2,045		1,500
Recorded investment	28,282		28,735
Unpaid principal balance	37,163		38,787
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,068	17,720
Impaired Financing Receivable, Interest Income, Accrual Method	3	66
Related allowance	1,012		797
Recorded investment	3,317		3,042
Unpaid principal balance	3,479		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	2,707	8,130
Impaired Financing Receivable, Interest Income, Accrual Method	13	10
Recorded investment	2,599		8,735
Unpaid principal balance	9,100		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
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	0		0
Unpaid principal balance	0		0
Originated Loans [Member] | Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	10,798	10,401
Impaired Financing Receivable, Interest Income, Accrual Method	72	63
Recorded investment	10,545		10,726
Unpaid principal balance	12,264		12,571
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,994	5,510
Impaired Financing Receivable, Interest Income, Accrual Method	36	31
Recorded investment	3,752		3,599
Unpaid principal balance	4,042		3,812
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	324	354
Impaired Financing Receivable, Interest Income, Accrual Method	1	2
Recorded investment	339		281
Unpaid principal balance	530		408
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	17,823	24,395
Impaired Financing Receivable, Interest Income, Accrual Method	122	106
Recorded investment	17,235		23,341
Unpaid principal balance	25,936		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
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Originated Loans [Member] | Residential Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	604	706
Impaired Financing Receivable, Interest Income, Accrual Method	5	0
Related allowance	119		107
Recorded investment	713		486
Unpaid principal balance	713		495
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,278	1,960
Impaired Financing Receivable, Interest Income, Accrual Method	1	1
Related allowance	914		596
Recorded investment	7,017		1,866
Unpaid principal balance	7,035		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
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Originated Loans [Member] | With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	6,950	20,386
Impaired Financing Receivable, Interest Income, Accrual Method	9	$ 67
Related allowance	2,045		1,500
Recorded investment	11,047		5,394
Unpaid principal balance	$ 11,227		$ 5,554