Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Narrative) (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) contracts	Jun. 30, 2016 USD ($) contracts	Jun. 30, 2017 USD ($) contracts	Jun. 30, 2016 USD ($) contracts	Jun. 30, 2017 USD ($) Loan	Jun. 30, 2016 USD ($) Loan	Dec. 31, 2016 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number	4	2	8	13	16	16
Allowance for loan losses related to impaired loans	$ 5,633,000	$ 6,611,000	$ 5,633,000	$ 6,611,000	$ 5,633,000	$ 6,611,000	$ 6,377,000
TDRs, Additions	$ 2,210,000	$ 655,000	$ 3,723,000	$ 9,378,000	7,242,000	9,801,000
Weighted average extension term	54 months	36 months	36 months	6 months
Interest-only payments, terms		6 months
Weighted average stated interest rate, basis points	1.95%	2.75%	1.84%	0.30%
Loan forgiveness	$ 0	$ 300,000	$ 0	$ 300,000
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs	33,091,000	49,635,000	$ 33,091,000	49,635,000	33,091,000	$ 49,635,000	$ 41,708,000
TDRs, number | contracts			77
Allowance for loan losses related to impaired loans	953,000		$ 953,000		953,000
Interest income, passage of time	49,000	$ 135,000	104,000	$ 225,000
Residential Real Estate | Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Foreclosed residential properties	8,400,000		8,400,000		8,400,000
In process other real estate owned	$ 12,400,000		$ 12,400,000		$ 12,400,000