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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 4,844		$ 3,189
Impaired Financing Receivable, Interest Income, Accrual Method	1		1
Related allowance	521		521		$ 16
Recorded investment	5,024		5,024		587
Unpaid principal balance	6,287		6,287		661
Acquired Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	786		316
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Related allowance	472		472		0
Recorded investment	934		934		0
Unpaid principal balance	940		940		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		48
Impaired Financing Receivable, Interest Income, Accrual Method	1		1
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	0		33
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	653		487
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Recorded investment	707		707		406
Unpaid principal balance	1,015		1,015		480
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,078		2,076
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Recorded investment	3,047		3,047		162
Unpaid principal balance	3,961		3,961		162
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	6		2
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Recorded investment	17		17		3
Unpaid principal balance	17		17		3
Acquired Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,809		2,646
Impaired Financing Receivable, Interest Income, Accrual Method	1		1
Recorded investment	3,843		3,843		571
Unpaid principal balance	5,065		5,065		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	95		68
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Related allowance	5		5		16
Recorded investment	94		94		16
Unpaid principal balance	123		123		16
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	154		159
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Related allowance	44		44		0
Recorded investment	153		153		0
Unpaid principal balance	159		159		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	1,035		543
Impaired Financing Receivable, Interest Income, Accrual Method	0		0
Related allowance	521		521		16
Recorded investment	1,181		1,181		16
Unpaid principal balance	1,222		1,222		16
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	33,819	$ 61,085	39,437	$ 59,083
Impaired Financing Receivable, Interest Income, Accrual Method	637	331	1,016	942
Related allowance	1,660		1,660		3,128
Recorded investment	33,778		33,778		41,226
Unpaid principal balance	47,151		47,151		56,296
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	8,510	17,207	8,984	15,310
Impaired Financing Receivable, Interest Income, Accrual Method	32	22	77	77
Related allowance	1,346		1,346		2,530
Recorded investment	8,394		8,394		13,423
Unpaid principal balance	12,631		12,631		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	7,106	24,674	11,627	24,020
Impaired Financing Receivable, Interest Income, Accrual Method	62	128	142	421
Recorded investment	7,028		7,028		9,549
Unpaid principal balance	13,464		13,464		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	6
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	11,217	11,636	11,417	11,546
Impaired Financing Receivable, Interest Income, Accrual Method	82	94	245	232
Recorded investment	10,970		10,970		10,873
Unpaid principal balance	12,810		12,810		13,004
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	5,928	6,463	6,439	7,143
Impaired Financing Receivable, Interest Income, Accrual Method	452	73	522	140
Recorded investment	6,203		6,203		5,765
Unpaid principal balance	6,958		6,958		6,905
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	366	384	350	423
Impaired Financing Receivable, Interest Income, Accrual Method	5	7	16	10
Recorded investment	357		357		382
Unpaid principal balance	417		417		507
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	24,617	43,157	29,833	43,138
Impaired Financing Receivable, Interest Income, Accrual Method	601	302	925	847
Recorded investment	24,558		24,558		26,569
Unpaid principal balance	33,649		33,649		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	377	174	266	111
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	97		97		164
Recorded investment	517		517		424
Unpaid principal balance	562		562		475
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	315	547	354	524
Impaired Financing Receivable, Interest Income, Accrual Method	4	7	14	18
Related allowance	217		217		434
Recorded investment	309		309		810
Unpaid principal balance	309		309		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	9,202	17,928	9,604	15,945
Impaired Financing Receivable, Interest Income, Accrual Method	36	$ 29	91	$ 95
Related allowance	1,660		1,660		3,128
Recorded investment	9,220		9,220		14,657
Unpaid principal balance	$ 13,502		$ 13,502		$ 20,511