Loans and Leases - Reserve for Loan and Lease Losses Roll Forward (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 17,499	$ 17,628	$ 20,662
Charge-offs	(7,137)	(6,988)	(8,496)
Recoveries	1,301	2,038	1,660
Provision (recovery of provision)	9,892	4,646	3,623
Provision (recovery of provision) for acquired credit impaired loans	0	175	179
Ending balance	21,555	17,499	17,628
Commercial, Financial and Agricultural [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,037	6,418	6,920
Charge-offs	(1,030)	(4,827)	(4,793)
Recoveries	801	1,454	1,032
Provision (recovery of provision)	(66)	3,992	3,259
Provision (recovery of provision) for acquired credit impaired loans	0	0	0
Ending balance	6,742	7,037	6,418
Real Estate-Commercial and Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,505	6,572	8,943
Charge-offs	(232)	(307)	(1,895)
Recoveries	5	101	200
Provision (recovery of provision)	2,561	961	(684)
Provision (recovery of provision) for acquired credit impaired loans	0	178	8
Ending balance	9,839	7,505	6,572
Real Estate-Residential Secured for Business Purpose [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	774	763	763
Charge-offs	(1,370)	(522)	(179)
Recoveries	54	71	28
Provision (recovery of provision)	2,204	462	43
Provision (recovery of provision) for acquired credit impaired loans	(1)	0	108
Ending balance	1,661	774	763
Real Estate-Residential and Home Equity Secured for Personal Purpose [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	993	1,575	1,124
Charge-offs	(196)	(178)	(279)
Recoveries	99	88	10
Provision (recovery of provision)	857	(489)	657
Provision (recovery of provision) for acquired credit impaired loans	1	(3)	63
Ending balance	1,754	993	1,575
Loans to Individuals [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	364	346	360
Charge-offs	(317)	(395)	(549)
Recoveries	136	133	176
Provision (recovery of provision)	190	280	359
Provision (recovery of provision) for acquired credit impaired loans	0	0	0
Ending balance	373	364	346
Lease Financings [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	788	1,042	985
Charge-offs	(3,992)	(759)	(801)
Recoveries	206	191	214
Provision (recovery of provision)	4,130	314	644
Provision (recovery of provision) for acquired credit impaired loans	0	0	0
Ending balance	1,132	788	1,042
Unallocated [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	38	912	1,567
Provision (recovery of provision)	16	(874)	(655)
Provision (recovery of provision) for acquired credit impaired loans	0	0	0
Ending balance	$ 54	$ 38	$ 912