Loans and Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 32,496	$ 28,719
Current Loans	4,975,022	4,693,276
Total Loans	5,007,518	4,721,995
Accruing Loans 90 Days or More Past Due	2,916	2,893
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,339	2,043
Current Loans	637,279	639,575
Total Loans	639,618	641,618
Accruing Loans 90 Days or More Past Due	118	84
Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,286	5,288
Current Loans	1,127,444	1,121,303
Total Loans	1,129,730	1,126,591
Accruing Loans 90 Days or More Past Due	171	432
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	6,196	3,591
Current Loans	323,487	326,743
Total Loans	329,683	330,334
Accruing Loans 90 Days or More Past Due	852	584
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	5,817	3,865
Current Loans	788,538	781,790
Total Loans	794,355	785,655
Accruing Loans 90 Days or More Past Due	1,129	973
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	6,077	4,701
Current Loans	1,298,337	1,279,704
Total Loans	1,304,414	1,284,405
Accruing Loans 90 Days or More Past Due	198	403
Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,329	3,048
Current Loans	298,798	285,872
Total Loans	301,127	288,920
Accruing Loans 90 Days or More Past Due	331	194
Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	973	4,525
Current Loans	136,378	139,920
Total Loans	137,351	144,445
Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	6,479	1,658
Current Loans	364,761	118,369
Total Loans	371,240	120,027
Accruing Loans 90 Days or More Past Due	117	223
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	14,746	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	547	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	475	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	4,333	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	2,414	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	1,768	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	1,549	2,085
30 to 59 Days Past Due [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	412	506
30 to 59 Days Past Due [Member] | Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,248	753
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	5,214	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	824	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	56	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	644	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	754	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	1,659	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	359	670
60 to 89 Days Past Due [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	237	103
60 to 89 Days Past Due [Member] | Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	681	192
90 Days and Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	12,536	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	968	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	1,755	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	1,219	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	2,649	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	2,650	1,785
90 Days and Greater [Member] | Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	421	293
90 Days and Greater [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	324	3,916
90 Days and Greater [Member] | Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 2,550	$ 713