Loans - Assigned Risk Rating within 12 Months of Balance Sheet Date (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	$ 1,511,008	$ 1,349,203
Commercial, Industrial, and Agricultural
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	808,565	749,138
Commercial mortgages
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	702,443	600,065
Pass
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	1,457,609	1,294,733
Pass | Commercial, Industrial, and Agricultural
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	776,022	713,102
Pass | Commercial mortgages
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	681,587	581,631
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	17,216	21,145
Special Mention | Commercial, Industrial, and Agricultural
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	11,255	16,726
Special Mention | Commercial mortgages
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	5,961	4,419
Substandard
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	36,183	33,325
Substandard | Commercial, Industrial, and Agricultural
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	21,288	19,310
Substandard | Commercial mortgages
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	14,895	14,015
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	0	0
Doubtful | Commercial, Industrial, and Agricultural
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	0	0
Doubtful | Commercial mortgages
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	$ 0	$ 0