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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 4,585	$ 4,844	$ 3,614	$ 3,189
Impaired Financing Receivable, Interest Income, Accrual Method	12	1	13	1
Related allowance	0		0		$ 33
Recorded investment	4,444		4,444		2,289
Unpaid principal balance	6,015		6,015		3,545
Acquired Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	786	0	316
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 Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	73	72	261	48
Impaired Financing Receivable, Interest Income, Accrual Method	10	1	10	1
Recorded investment	73		73		436
Unpaid principal balance	73		73		449
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	33
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	2,541	653	1,779	487
Impaired Financing Receivable, Interest Income, Accrual Method	2	0	3	0
Recorded investment	2,436		2,436		666
Unpaid principal balance	3,006		3,006		965
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,955	3,078	1,558	2,076
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	1,920		1,920		940
Unpaid principal balance	2,919		2,919		1,842
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	16	6	16	2
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	15		15		17
Unpaid principal balance	17		17		17
Acquired Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	4,585	3,809	3,614	2,646
Impaired Financing Receivable, Interest Income, Accrual Method	12	1	13	1
Recorded investment	4,444		4,444		2,059
Unpaid principal balance	6,015		6,015		3,273
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	95	0	68
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		4
Recorded investment	0		0		93
Unpaid principal balance	0		0		122
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	154	0	159
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		29
Recorded investment	0		0		137
Unpaid principal balance	0		0		150
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	0	1,035	0	543
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		33
Recorded investment	0		0		230
Unpaid principal balance	0		0		272
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	38,659	33,819	44,729	39,437
Impaired Financing Receivable, Interest Income, Accrual Method	135	637	949	1,016
Related allowance	5,363		5,363		3,713
Recorded investment	35,405		35,405		39,951
Unpaid principal balance	46,703		46,703		53,021
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	10,298	8,510	5,979	8,984
Impaired Financing Receivable, Interest Income, Accrual Method	4	32	16	77
Related allowance	3,474		3,474		3,478
Recorded investment	9,998		9,998		16,866
Unpaid principal balance	10,219		10,219		21,094
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	5,697	7,106	17,838	11,627
Impaired Financing Receivable, Interest Income, Accrual Method	8	62	576	142
Recorded investment	3,177		3,177		5,548
Unpaid principal balance	9,585		9,585		12,153
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	10,627	11,217	10,639	11,417
Impaired Financing Receivable, Interest Income, Accrual Method	59	82	191	245
Recorded investment	10,302		10,302		10,625
Unpaid principal balance	12,163		12,163		12,470
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	6,810	5,928	7,632	6,439
Impaired Financing Receivable, Interest Income, Accrual Method	59	452	146	522
Recorded investment	4,951		4,951		5,155
Unpaid principal balance	7,559		7,559		5,489
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	320	366	322	350
Impaired Financing Receivable, Interest Income, Accrual Method	2	5	6	16
Recorded investment	287		287		347
Unpaid principal balance	414		414		383
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	23,454	24,617	36,431	29,833
Impaired Financing Receivable, Interest Income, Accrual Method	128	601	919	925
Recorded investment	18,717		18,717		21,675
Unpaid principal balance	29,721		29,721		30,495
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	565	377	532	266
Impaired Financing Receivable, Interest Income, Accrual Method	2	0	11	0
Related allowance	167		167		107
Recorded investment	596		596		456
Unpaid principal balance	602		602		478
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	4,342	315	1,787	354
Impaired Financing Receivable, Interest Income, Accrual Method	1	4	3	14
Related allowance	1,722		1,722		128
Recorded investment	6,094		6,094		954
Unpaid principal balance	6,161		6,161		954
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	15,205	9,202	8,298	9,604
Impaired Financing Receivable, Interest Income, Accrual Method	7	$ 36	30	$ 91
Related allowance	5,363		5,363		3,713
Recorded investment	16,688		16,688		18,276
Unpaid principal balance	$ 16,982		$ 16,982		$ 22,526