Loans and Leases - Non-Performing Loans and Leases (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) borrowers	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Non-Performing Loans and Leases [Line Items]
Nonaccrual Loans and Leases		$ 14,517	$ 17,916
Accruing Troubled Debt Restructured Loans and Lease Modifications		11,435	3,252
Loans and Leases 90 Days or more Past Due and Accruing Interest		761	987
Total Nonperforming Loans and Leases		26,713	22,155
Nonaccrual Troubled Debt Restructured Loans And Lease Modifications		2,500	1,800
Number of Borrowers | borrowers	1
Commercial, Financial and Agricultural [Member]
Non-Performing Loans and Leases [Line Items]
Nonaccrual Loans and Leases		4,448	5,746
Accruing Troubled Debt Restructured Loans and Lease Modifications		921	967
Loans and Leases 90 Days or more Past Due and Accruing Interest		0	0
Total Nonperforming Loans and Leases		5,369	6,713
Real Estate-Commercial [Member]
Non-Performing Loans and Leases [Line Items]
Nonaccrual Loans and Leases		4,285	5,651
Accruing Troubled Debt Restructured Loans and Lease Modifications		10,266	1,519
Loans and Leases 90 Days or more Past Due and Accruing Interest		0	0
Total Nonperforming Loans and Leases		14,551	7,170
Real Estate-Construction [Member]
Non-Performing Loans and Leases [Line Items]
Nonaccrual Loans and Leases		365	0
Accruing Troubled Debt Restructured Loans and Lease Modifications		0	0
Loans and Leases 90 Days or more Past Due and Accruing Interest		0	0
Total Nonperforming Loans and Leases		365	0
Real Estate-Residential Secured for Business Purpose [Member]
Non-Performing Loans and Leases [Line Items]
Nonaccrual Loans and Leases		2,843	4,898
Accruing Troubled Debt Restructured Loans and Lease Modifications		206	766
Loans and Leases 90 Days or more Past Due and Accruing Interest		162	0
Total Nonperforming Loans and Leases		3,211	5,664
Real Estate-Residential Secured for Personal Purpose [Member]
Non-Performing Loans and Leases [Line Items]
Nonaccrual Loans and Leases		466	560
Accruing Troubled Debt Restructured Loans and Lease Modifications		42	0
Loans and Leases 90 Days or more Past Due and Accruing Interest		0	481
Total Nonperforming Loans and Leases		508	1,041
Real Estate-Home Equity Secured for Personal Purpose [Member]
Non-Performing Loans and Leases [Line Items]
Nonaccrual Loans and Leases		511	525
Accruing Troubled Debt Restructured Loans and Lease Modifications		0	0
Loans and Leases 90 Days or more Past Due and Accruing Interest		148	171
Total Nonperforming Loans and Leases		659	696
Loans to Individuals [Member]
Non-Performing Loans and Leases [Line Items]
Nonaccrual Loans and Leases		0	0
Accruing Troubled Debt Restructured Loans and Lease Modifications		0	0
Loans and Leases 90 Days or more Past Due and Accruing Interest		195	142
Total Nonperforming Loans and Leases		195	142
Lease Financings [Member]
Non-Performing Loans and Leases [Line Items]
Nonaccrual Loans and Leases		1,599	536
Accruing Troubled Debt Restructured Loans and Lease Modifications		0	0
Loans and Leases 90 Days or more Past Due and Accruing Interest		256	193
Total Nonperforming Loans and Leases		1,855	729
Accrual Troubled Debt Restructuring Loans [Member]
Non-Performing Loans and Leases [Line Items]
Pre- Restructuring Outstanding Recorded Investment		9,206	1,960
Accrual Troubled Debt Restructuring Loans [Member] | Commercial, Financial and Agricultural [Member]
Non-Performing Loans and Leases [Line Items]
Pre- Restructuring Outstanding Recorded Investment		0	1,545
Accrual Troubled Debt Restructuring Loans [Member] | Real Estate-Commercial [Member]
Non-Performing Loans and Leases [Line Items]
Pre- Restructuring Outstanding Recorded Investment		9,206	0
Accrual Troubled Debt Restructuring Loans [Member] | Real Estate-Residential Secured for Business Purpose [Member]
Non-Performing Loans and Leases [Line Items]
Pre- Restructuring Outstanding Recorded Investment		$ 0	$ 415
Amortization Period Extension [Member] | Accrual Troubled Debt Restructuring Loans [Member]
Non-Performing Loans and Leases [Line Items]
Pre- Restructuring Outstanding Recorded Investment	$ 9,200