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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 4,284		$ 2,376
Impaired Financing Receivable, Interest Income, Accrual Method	7		7
Related allowance	164		164		$ 16
Recorded investment	4,498		4,498		587
Unpaid principal balance	5,707		5,707		661
Acquired Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	164		82
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Related allowance	110		110		0
Recorded investment	490		490		0
Unpaid principal balance	490		490		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	72		36
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Recorded investment	72		72		0
Unpaid principal balance	72		72		0
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	100		50
Impaired Financing Receivable, Interest Income, Accrual Method	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	623		465
Impaired Financing Receivable, Interest Income, Accrual Method	7		7
Recorded investment	624		624		406
Unpaid principal balance	943		943		480
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,151		1,575
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Recorded investment	3,140		3,140		162
Unpaid principal balance	4,012		4,012		162
Acquired Loans [Member] | Loans To Individuals 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		3
Unpaid principal balance	0		0		3
Acquired Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,946		2,126
Impaired Financing Receivable, Interest Income, Accrual Method	7		7
Recorded investment	3,836		3,836		571
Unpaid principal balance	5,027		5,027		645
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		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	14		7
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Related allowance	4		4		16
Recorded investment	14		14		16
Unpaid principal balance	28		28		16
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	160		161
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Related allowance	50		50		0
Recorded investment	158		158		0
Unpaid principal balance	162		162		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		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	338		250
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Related allowance	164		164		16
Recorded investment	662		662		16
Unpaid principal balance	680		680		16
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	40,488	$ 61,934	42,232	$ 58,082
Impaired Financing Receivable, Interest Income, Accrual Method	430	287	632	602
Related allowance	1,971		1,971		3,128
Recorded investment	35,687		35,687		41,226
Unpaid principal balance	49,146		49,146		56,296
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	8,813	22,788	9,220	17,939
Impaired Financing Receivable, Interest Income, Accrual Method	20	65	46	91
Related allowance	1,681		1,681		2,530
Recorded investment	8,634		8,634		13,423
Unpaid principal balance	12,699		12,699		19,226
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	12,583	18,995	13,888	20,116
Impaired Financing Receivable, Interest Income, Accrual Method	251	95	289	249
Recorded investment	8,290		8,290		9,549
Unpaid principal balance	14,697		14,697		15,369
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	9
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	44
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	11,339	11,462	11,434	11,232
Impaired Financing Receivable, Interest Income, Accrual Method	110	90	185	137
Recorded investment	11,128		11,128		10,873
Unpaid principal balance	12,975		12,975		13,004
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	6,596	6,887	6,586	7,136
Impaired Financing Receivable, Interest Income, Accrual Method	34	29	88	67
Recorded investment	6,524		6,524		5,765
Unpaid principal balance	7,498		7,498		6,905
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	344	405	341	442
Impaired Financing Receivable, Interest Income, Accrual Method	9	2	11	3
Recorded investment	341		341		382
Unpaid principal balance	391		391		507
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	30,862	37,749	32,249	38,935
Impaired Financing Receivable, Interest Income, Accrual Method	404	216	573	500
Recorded investment	26,283		26,283		26,569
Unpaid principal balance	35,561		35,561		35,785
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	332	476	279	347
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	31		31		164
Recorded investment	342		342		424
Unpaid principal balance	459		459		475
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	481	921	484	861
Impaired Financing Receivable, Interest Income, Accrual Method	6	6	13	11
Related allowance	259		259		434
Recorded investment	428		428		810
Unpaid principal balance	427		427		810
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	9,626	24,185	9,983	19,147
Impaired Financing Receivable, Interest Income, Accrual Method	26	$ 71	59	$ 102
Related allowance	1,971		1,971		3,128
Recorded investment	9,404		9,404		14,657
Unpaid principal balance	$ 13,585		$ 13,585		$ 20,511