Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Narrative) (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) contracts	Sep. 30, 2016 USD ($) contracts	Sep. 30, 2017 USD ($) contracts	Sep. 30, 2016 USD ($) contracts	Sep. 30, 2017 USD ($) Loan	Sep. 30, 2016 USD ($) Loan	Dec. 31, 2016 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number	5	2	13	15	19	17
Allowance for loan losses related to impaired loans	$ 7,218,000	$ 6,836,000	$ 7,218,000	$ 6,836,000	$ 7,218,000	$ 6,836,000	$ 6,377,000
TDRs, Additions	$ 1,663,000	$ 71,000	$ 5,386,000	$ 9,449,000	8,834,000	9,650,000
Weighted average extension term	36 months	22 months	36 months	6 months
Weighted average stated interest rate, basis points	2.25%	1.50%	1.88%	0.30%
Loan forgiveness	$ 73,000	$ 0	$ 73,000	$ 300,000
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs	33,183,000	44,276,000	$ 33,183,000	44,276,000	33,183,000	$ 44,276,000	$ 41,708,000
TDRs, number | contracts			78
Allowance for loan losses related to impaired loans	1,200,000		$ 1,200,000		1,200,000
Interest income, passage of time	68,000	$ 98,000	172,000	$ 323,000
Residential Real Estate | Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Foreclosed residential properties	7,900,000		7,900,000		7,900,000
In process other real estate owned	$ 12,100,000		$ 12,100,000		$ 12,100,000