Loans - Schedule of Loan Assigned Risk Rating within 12 Months of Balance Sheet Date (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	$ 2,454,853	$ 2,126,266
Commercial, industrial, and agricultural
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	916,297	704,606
Commercial mortgages
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	697,776	644,597
Pass
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	1,572,960	1,294,733
Pass | Commercial, industrial, and agricultural
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	889,547	668,570
Pass | Commercial mortgages
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	683,413	626,163
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	13,760	21,145
Special Mention | Commercial, industrial, and agricultural
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	10,519	16,726
Special Mention | Commercial mortgages
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	3,241	4,419
Substandard
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	27,353	33,325
Substandard | Commercial, industrial, and agricultural
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	16,231	19,310
Substandard | Commercial mortgages
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	11,122	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
Total
Financing Receivable, Recorded Investment [Line Items]
Financing receivable recorded investment	$ 1,614,073	$ 1,349,203