Loans and Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 40,980	$ 28,719
Current Loans	4,906,548	4,693,276
Total Loans	4,947,528	4,721,995
Accruing Loans 90 Days or More Past Due	4,073	2,893
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,894	2,043
Current Loans	627,528	639,575
Total Loans	630,422	641,618
Accruing Loans 90 Days or More Past Due	90	84
Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	5,521	5,288
Current Loans	1,111,929	1,121,303
Total Loans	1,117,450	1,126,591
Accruing Loans 90 Days or More Past Due	57	432
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	6,189	3,591
Current Loans	322,237	326,743
Total Loans	328,426	330,334
Accruing Loans 90 Days or More Past Due	427	584
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	7,157	3,865
Current Loans	810,727	781,790
Total Loans	817,884	785,655
Accruing Loans 90 Days or More Past Due	845	973
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	6,264	4,701
Current Loans	1,263,816	1,279,704
Total Loans	1,270,080	1,284,405
Accruing Loans 90 Days or More Past Due	1,070	403
Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,803	3,048
Current Loans	317,629	285,872
Total Loans	320,432	288,920
Accruing Loans 90 Days or More Past Due	345	194
Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	886	4,525
Current Loans	142,319	139,920
Total Loans	143,205	144,445
Accruing Loans 90 Days or More Past Due	128
Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	9,266	1,658
Current Loans	310,363	118,369
Total Loans	319,629	120,027
Accruing Loans 90 Days or More Past Due	1,111	223
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	19,620	13,407
30 to 59 Days Past Due [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	977	998
30 to 59 Days Past Due [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	4,964	2,905
30 to 59 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,910	2,211
30 to 59 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,034	1,739
30 to 59 Days Past Due [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,573	2,210
30 to 59 Days Past Due [Member] | Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,721	2,085
30 to 59 Days Past Due [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	514	506
30 to 59 Days Past Due [Member] | Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,927	753
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	9,773	3,162
60 to 89 Days Past Due [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,106	68
60 to 89 Days Past Due [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	385	271
60 to 89 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,318	403
60 to 89 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,790	749
60 to 89 Days Past Due [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,469	706
60 to 89 Days Past Due [Member] | Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	530	670
60 to 89 Days Past Due [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		103
60 to 89 Days Past Due [Member] | Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,175	192
90 Days and Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	11,587	12,150
90 Days and Greater [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	811	977
90 Days and Greater [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	172	2,112
90 Days and Greater [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	961	977
90 Days and Greater [Member] | Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,333	1,377
90 Days and Greater [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,222	1,785
90 Days and Greater [Member] | Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	552	293
90 Days and Greater [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	372	3,916
90 Days and Greater [Member] | Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 4,164	$ 713