Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans 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, 2015 USD ($) contracts		Jun. 30, 2014 USD ($) contracts		Jun. 30, 2015 USD ($) contracts		Jun. 30, 2014 USD ($) contracts		Jun. 30, 2015 USD ($) contracts		Jun. 30, 2014 USD ($) contracts		Dec. 31, 2014 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number | contracts		6	[1],[2]	7	[1],[2]	9	[1],[2]	13	[1],[2]	17	[3],[4]	22	[3],[4]
Allowance for loan losses related to impaired loans		$ 10,075,000		$ 10,298,000		$ 10,075,000		$ 10,298,000		$ 10,075,000		$ 10,298,000		$ 6,270,000
TDRs		$ 1,317,000	[1],[2]	$ 2,240,000	[1],[2]	$ 1,611,000	[1],[2]	$ 7,485,000	[1],[2]	6,093,000	[3],[4]	13,420,000	[3],[4]
Weighted average extension term		29 months		16 months		27 months		14 months
Weighted average stated interest rate, basis points		4.08%		1.37%		3.67%		1.67%
Weighted average interest only term		29 months		6 months		28 months		9 months
Loan forgiveness		$ 0		$ 0		$ 0		$ 0
Modification to Interest Only Payments
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number | contracts	[1]	1		1		2		4
TDRs	[1]	$ 169,000		$ 220,000		$ 219,000		$ 2,659,000
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Restructured loans		62,776,000		88,107,000		$ 62,776,000		88,107,000		62,776,000		$ 88,107,000		$ 82,275,000
TDRs, number | contracts						122
Allowance for loan losses related to impaired loans		3,700,000				$ 3,700,000				3,700,000
Interest income, passage of time		94,000		$ 103,000		287,000		$ 235,000
Residential Real Estate [Member] | Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Repossessed Assets		$ 9,400,000				$ 9,400,000				$ 9,400,000

[1] Balances represent the recorded investment in the loan at the time of the restructuring.

[2] TDRs may have more than one modification representing a concession. As such, TDRs during the period may be represented in more than one of the categories noted above.

[3] Balances represent the recorded investment in the loan at the time of the restructuring.

[4] Total TDRs represent all loans restructured in TDRs during the previous twelve months from the date indicated.