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, 2018	Mar. 31, 2017	Dec. 31, 2017
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 2,224	$ 468
Impaired Financing Receivable, Interest Income, Accrual Method	1	0
Related allowance	12		$ 33
Recorded investment	2,198		2,289
Unpaid principal balance	3,501		3,545
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		0
Recorded investment	0		0
Unpaid principal balance	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	411	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	411		436
Unpaid principal balance	424		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
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	678	241
Impaired Financing Receivable, Interest Income, Accrual Method	1	0
Recorded investment	677		666
Unpaid principal balance	1,003		965
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	907	162
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	891		940
Unpaid principal balance	1,811		1,842
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	17	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	17		17
Unpaid principal balance	17		17
Acquired Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	2,013	403
Impaired Financing Receivable, Interest Income, Accrual Method	1	0
Recorded investment	1,996		2,059
Unpaid principal balance	3,255		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
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	93	65
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	10		4
Recorded investment	93		93
Unpaid principal balance	122		122
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	118	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	2		29
Recorded investment	109		137
Unpaid principal balance	124		150
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	211	65
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	12		33
Recorded investment	202		230
Unpaid principal balance	246		272
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	44,781	43,975
Impaired Financing Receivable, Interest Income, Accrual Method	173	202
Related allowance	11,461		3,713
Recorded investment	55,059		39,951
Unpaid principal balance	62,380		53,021
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	17,720	12,787
Impaired Financing Receivable, Interest Income, Accrual Method	66	26
Related allowance	9,045		3,478
Recorded investment	28,615		16,866
Unpaid principal balance	28,795		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	8,130	12,034
Impaired Financing Receivable, Interest Income, Accrual Method	10	38
Recorded investment	4,791		5,548
Unpaid principal balance	9,559		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
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	10,401	11,422
Impaired Financing Receivable, Interest Income, Accrual Method	63	74
Recorded investment	10,129		10,625
Unpaid principal balance	12,000		12,470
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	5,510	5,949
Impaired Financing Receivable, Interest Income, Accrual Method	31	54
Recorded investment	4,624		5,155
Unpaid principal balance	5,038		5,489
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	354	338
Impaired Financing Receivable, Interest Income, Accrual Method	2	2
Recorded investment	339		347
Unpaid principal balance	379		383
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	24,395	29,743
Impaired Financing Receivable, Interest Income, Accrual Method	106	168
Recorded investment	19,883		21,675
Unpaid principal balance	26,976		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
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	706	334
Impaired Financing Receivable, Interest Income, Accrual Method	0	1
Related allowance	277		107
Recorded investment	808		456
Unpaid principal balance	846		478
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,960	1,111
Impaired Financing Receivable, Interest Income, Accrual Method	1	7
Related allowance	2,139		128
Recorded investment	5,753		954
Unpaid principal balance	5,763		954
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	20,386	14,232
Impaired Financing Receivable, Interest Income, Accrual Method	67	$ 34
Related allowance	11,461		3,713
Recorded investment	35,176		18,276
Unpaid principal balance	$ 35,404		$ 22,526