Loans and Leases - Additional Allowance for Credit Losses Relating to Acquired Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and leases receivable, allowance	$ 175,380	$ 158,059	$ 142,012	$ 125,926
Acquired Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and leases receivable, allowance	6,698	7,268	$ 6,727	$ 7,974
Acquired Loans | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and leases receivable, allowance	4,976	4,538
Acquired Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and leases receivable, allowance	(415)	500
Acquired Loans | Total commercial loans and leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and leases receivable, allowance	4,561	5,038
Acquired Loans | Direct installment
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and leases receivable, allowance	1,553	1,005
Acquired Loans | Residential mortgages
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and leases receivable, allowance	484	632
Acquired Loans | Indirect installment
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and leases receivable, allowance	177	221
Acquired Loans | Consumer lines of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and leases receivable, allowance	(77)	372
Acquired Loans | Total consumer loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and leases receivable, allowance	$ 2,137	$ 2,230