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, 2017	Mar. 31, 2016	Dec. 31, 2016
Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Related allowance
Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Related allowance
Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Related allowance
Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Related allowance
Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Related allowance
With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Related allowance
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	468
Impaired Financing Receivable, Interest Income, Accrual Method	0
Related allowance	29		16
Recorded investment	473		587
Unpaid principal balance	548		661
Acquired Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0
Impaired Financing Receivable, Interest Income, Accrual Method	0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0
Impaired Financing Receivable, Interest Income, Accrual Method	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	241
Impaired Financing Receivable, Interest Income, Accrual Method	0
Recorded investment	246		406
Unpaid principal balance	321		480
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	162
Impaired Financing Receivable, Interest Income, Accrual Method	0
Recorded investment	162		162
Unpaid principal balance	162		162
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0
Impaired Financing Receivable, Interest Income, Accrual Method	0
Recorded investment	0		3
Unpaid principal balance	0		3
Acquired Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	403
Impaired Financing Receivable, Interest Income, Accrual Method	0
Recorded investment	408		571
Unpaid principal balance	483		645
Acquired Loans [Member] | Commercial Financial Agricultural And Other With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0
Impaired Financing Receivable, Interest Income, Accrual Method	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Real Estate Construction With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0
Impaired Financing Receivable, Interest Income, Accrual Method	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	65
Impaired Financing Receivable, Interest Income, Accrual Method	0
Related allowance	29		16
Recorded investment	65		16
Unpaid principal balance	65		16
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0
Impaired Financing Receivable, Interest Income, Accrual Method	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Loans To Individuals With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0
Impaired Financing Receivable, Interest Income, Accrual Method	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	65
Impaired Financing Receivable, Interest Income, Accrual Method	0
Related allowance	29		16
Recorded investment	65		16
Unpaid principal balance	65		16
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	43,975	$ 54,230
Impaired Financing Receivable, Interest Income, Accrual Method	202	315
Related allowance	2,988		3,128
Recorded investment	45,796		41,226
Unpaid principal balance	62,389		56,296
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	12,034	17,298
Impaired Financing Receivable, Interest Income, Accrual Method	38	152
Recorded investment	12,780		9,549
Unpaid principal balance	20,043		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	17
Impaired Financing Receivable, Interest Income, Accrual Method	0	44
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	11,422	10,724
Impaired Financing Receivable, Interest Income, Accrual Method	74	47
Recorded investment	11,227		10,873
Unpaid principal balance	13,237		13,004
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	5,949	7,658
Impaired Financing Receivable, Interest Income, Accrual Method	54	38
Recorded investment	6,028		5,765
Unpaid principal balance	7,037		6,905
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	338	480
Impaired Financing Receivable, Interest Income, Accrual Method	2	1
Recorded investment	365		382
Unpaid principal balance	429		507
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	29,743	36,177
Impaired Financing Receivable, Interest Income, Accrual Method	168	282
Recorded investment	30,400		26,569
Unpaid principal balance	40,746		35,785
Originated Loans [Member] | Commercial Financial Agricultural And Other With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	12,787	17,027
Impaired Financing Receivable, Interest Income, Accrual Method	26	28
Related allowance	2,466		2,530
Recorded investment	13,929		13,423
Unpaid principal balance	20,124		19,226
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	334	497
Impaired Financing Receivable, Interest Income, Accrual Method	1	0
Related allowance	146		164
Recorded investment	366		424
Unpaid principal balance	418		475
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,111	529
Impaired Financing Receivable, Interest Income, Accrual Method	7	5
Related allowance	376		434
Recorded investment	1,101		810
Unpaid principal balance	1,101		810
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	14,232	18,053
Impaired Financing Receivable, Interest Income, Accrual Method	34	$ 33
Related allowance	2,988		3,128
Recorded investment	15,396		14,657
Unpaid principal balance	$ 21,643		$ 20,511