Loans and Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 28,719	$ 35,200
Current Loans	4,693,276	4,365,032
Total Loans	4,721,995	4,400,232
Accruing Loans 90 Days or More Past Due	2,893	1,962
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	13,407	17,652
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,162	4,948
90 Days and Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	12,150	12,600
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,043	2,501
Current Loans	639,575	569,130
Total Loans	641,618	571,631
Accruing Loans 90 Days or More Past Due	84
Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	5,288	1,045
Current Loans	1,121,303	1,122,351
Total Loans	1,126,591	1,123,396
Accruing Loans 90 Days or More Past Due	432
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,591	4,664
Current Loans	326,743	320,749
Total Loans	330,334	325,413
Accruing Loans 90 Days or More Past Due	584	513
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,865	4,506
Current Loans	781,790	743,723
Total Loans	785,655	748,229
Accruing Loans 90 Days or More Past Due	973	369
Real Estate [Member] | 30 to 59 Days Past Due [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	998	2,255
Real Estate [Member] | 30 to 59 Days Past Due [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,905	611
Real Estate [Member] | 30 to 59 Days Past Due [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,211	2,742
Real Estate [Member] | 30 to 59 Days Past Due [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,739	2,559
Real Estate [Member] | 60 to 89 Days Past Due [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	68	96
Real Estate [Member] | 60 to 89 Days Past Due [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	271	16
Real Estate [Member] | 60 to 89 Days Past Due [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	403	649
Real Estate [Member] | 60 to 89 Days Past Due [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	749	531
Real Estate [Member] | 90 Days and Greater [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	977	150
Real Estate [Member] | 90 Days and Greater [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,112	418
Real Estate [Member] | 90 Days and Greater [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	977	1,273
Real Estate [Member] | 90 Days and Greater [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,377	1,416
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	4,701	3,638
Current Loans	1,279,704	1,047,547
Total Loans	1,284,405	1,051,185
Accruing Loans 90 Days or More Past Due	403	608
Commercial and Financial [Member] | 30 to 59 Days Past Due [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,210	1,269
Commercial and Financial [Member] | 60 to 89 Days Past Due [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	706	1,628
Commercial and Financial [Member] | 90 Days and Greater [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,785	741
Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,048	3,225
Current Loans	285,872	280,652
Total Loans	288,920	283,877
Accruing Loans 90 Days or More Past Due	194	274
Consumer Non-real Estate [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,085	2,046
Consumer Non-real Estate [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	670	760
Consumer Non-real Estate [Member] | 90 Days and Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	293	419
Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	4,525	14,078
Current Loans	139,920	127,404
Total Loans	144,445	141,482
Accruing Loans 90 Days or More Past Due		45
Other Loans [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	506	5,345
Other Loans [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	103	958
Other Loans [Member] | 90 Days and Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,916	7,775
Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,658	1,543
Current Loans	118,369	153,476
Total Loans	120,027	155,019
Accruing Loans 90 Days or More Past Due	223	153
Acquired Loans [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	753	825
Acquired Loans [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	192	310
Acquired Loans [Member] | 90 Days and Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 713	$ 408