Loans - Schedule of Loan Assigned Risk Rating within 12 Months of Balance Sheet Date (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	$ 1,349,203	$ 1,142,626
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	749,138	567,800
Commercial Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	600,065	574,826
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	1,294,733	1,082,794
Pass [Member] | Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	713,102	531,320
Pass [Member] | Commercial Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	581,631	551,474
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	21,145	16,447
Special Mention [Member] | Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	16,726	14,638
Special Mention [Member] | Commercial Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	4,419	1,809
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	33,325	43,374
Substandard [Member] | Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	19,310	21,831
Substandard [Member] | Commercial Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	14,015	21,543
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	0	11
Doubtful [Member] | Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	0	11
Doubtful [Member] | Commercial Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	$ 0	$ 0