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, 2016 USD ($) contracts Rate	Jun. 30, 2015 USD ($) contracts Rate	Jun. 30, 2016 USD ($) contracts Rate	Jun. 30, 2015 USD ($) contracts Rate	Jun. 30, 2016 USD ($) Loan	Jun. 30, 2015 USD ($) Loan	Dec. 31, 2015 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number	2	6	13	9	16	17
Allowance for loan losses related to impaired loans	$ 6,611,000	$ 10,075,000	$ 6,611,000	$ 10,075,000	$ 6,611,000	$ 10,075,000	$ 6,380,000
TDRs, Additions	$ 655,000	$ 1,317,000	$ 9,378,000	$ 1,611,000	9,801,000	6,093,000
Weighted average extension term	36 months	29 months	6 months	27 months
Weighted average stated interest rate, basis points | Rate	2.75%	4.08%	0.30%	3.67%
Weighted average interest only term	6 months	29 months	6 months	28 months
Loan forgiveness	$ 300,000	$ 0	$ 300,000	$ 0
Modification to Interest Only Payments
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number | contracts	1	1	1	2
TDRs, Additions	$ 380,000	$ 169,000	$ 380,000	$ 219,000
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs	49,635,000	62,776,000	$ 49,635,000	62,776,000	49,635,000	$ 62,776,000	$ 51,853,000
TDRs, number | contracts			97
Allowance for loan losses related to impaired loans	3,200,000		$ 3,200,000		3,200,000
Interest income, passage of time	135,000	$ 94,000	225,000	$ 287,000
Residential Real Estate | Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Foreclosed residential properties	$ 11,300,000		$ 11,300,000		$ 11,300,000