Loans - Schedule of Loan Assigned Risk Rating within 12 Months of Balance Sheet Date (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	$ 1,467,990	$ 1,349,203
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	773,473	749,138
Commercial Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	694,517	600,065
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	1,417,246	1,294,733
Pass [Member] | Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	740,558	713,102
Pass [Member] | Commercial Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	676,688	581,631
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	12,284	21,145
Special Mention [Member] | Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	9,513	16,726
Special Mention [Member] | Commercial Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	2,771	4,419
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	38,460	33,325
Substandard [Member] | Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	23,402	19,310
Substandard [Member] | Commercial Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	15,058	14,015
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	0	0
Doubtful [Member] | Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	0	0
Doubtful [Member] | Commercial Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	$ 0	$ 0