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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 4,034	$ 4,284	$ 3,129	$ 2,376
Impaired Financing Receivable, Interest Income, Accrual Method	1	7	1	7
Related allowance	61		61		$ 33
Recorded investment	4,750		4,750		2,289
Unpaid principal balance	6,252		6,252		3,545
Acquired Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	164	0	82
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	299	72	355	36
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
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	100	0	50
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	1,962	623	1,366	465
Impaired Financing Receivable, Interest Income, Accrual Method	1	7	1	7
Recorded investment	2,534		2,534		666
Unpaid principal balance	3,056		3,056		965
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,692	3,151	1,359	1,575
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	2,030		2,030		940
Unpaid principal balance	2,994		2,994		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	0	17	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	16		16		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	3,969	3,946	3,097	2,126
Impaired Financing Receivable, Interest Income, Accrual Method	1	7	1	7
Recorded investment	4,653		4,653		2,059
Unpaid principal balance	6,140		6,140		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	65	14	32	7
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	61		61		4
Recorded investment	97		97		93
Unpaid principal balance	112		112		122
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	160	0	161
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	65	338	32	250
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	61		61		33
Recorded investment	97		97		230
Unpaid principal balance	112		112		272
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	50,748	40,488	47,764	42,232
Impaired Financing Receivable, Interest Income, Accrual Method	633	430	807	632
Related allowance	7,540		7,540		3,713
Recorded investment	41,113		41,113		39,951
Unpaid principal balance	50,880		50,880		53,021
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	22,812	8,813	19,694	9,220
Impaired Financing Receivable, Interest Income, Accrual Method	23	20	52	46
Related allowance	6,597		6,597		3,478
Recorded investment	19,537		19,537		16,866
Unpaid principal balance	19,889		19,889		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	6,793	12,583	8,034	13,888
Impaired Financing Receivable, Interest Income, Accrual Method	481	251	528	289
Recorded investment	1,932		1,932		5,548
Unpaid principal balance	6,769		6,769		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	10,334	11,339	10,316	11,434
Impaired Financing Receivable, Interest Income, Accrual Method	64	110	125	185
Recorded investment	10,347		10,347		10,625
Unpaid principal balance	12,174		12,174		12,470
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	6,086	6,596	6,076	6,586
Impaired Financing Receivable, Interest Income, Accrual Method	23	34	54	88
Recorded investment	4,463		4,463		5,155
Unpaid principal balance	7,073		7,073		5,489
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	292	344	322	341
Impaired Financing Receivable, Interest Income, Accrual Method	3	9	4	11
Recorded investment	253		253		347
Unpaid principal balance	292		292		383
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	23,505	30,862	24,748	32,249
Impaired Financing Receivable, Interest Income, Accrual Method	571	404	711	573
Recorded investment	16,995		16,995		21,675
Unpaid principal balance	26,308		26,308		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	880	332	845	279
Impaired Financing Receivable, Interest Income, Accrual Method	5	0	9	0
Related allowance	281		281		107
Recorded investment	877		877		456
Unpaid principal balance	948		948		478
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,551	481	2,477	484
Impaired Financing Receivable, Interest Income, Accrual Method	34	6	35	13
Related allowance	662		662		128
Recorded investment	3,704		3,704		954
Unpaid principal balance	3,735		3,735		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	27,243	9,626	23,016	9,983
Impaired Financing Receivable, Interest Income, Accrual Method	62	$ 26	96	$ 59
Related allowance	7,540		7,540		3,713
Recorded investment	24,118		24,118		18,276
Unpaid principal balance	$ 24,572		$ 24,572		$ 22,526