Loans - Nonaccrual Loans and Loans Past Due over 90 Days Still Accruing Interest by Class of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual	$ 19,750	$ 19,232
Past Due Over 90 Days Still on Accrual	475	477
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual	2,737	1,869
Past Due Over 90 Days Still on Accrual	0	78
Commercial Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual	11,361	11,065
Past Due Over 90 Days Still on Accrual	0	0
Residential Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual	5,038	5,470
Past Due Over 90 Days Still on Accrual	425	338
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual	614	828
Past Due Over 90 Days Still on Accrual	13	17
Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual	0	0
Past Due Over 90 Days Still on Accrual	$ 37	$ 44