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) (USD $)	3 Months Ended				12 Months Ended
	Mar. 31, 2015 contracts		Mar. 31, 2014 contracts		Mar. 31, 2015 contracts		Mar. 31, 2014 contracts		Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number	3	[1],[2]	6	[1],[2]	18	[1],[3]	27	[1],[3]
Allowance for loan losses related to impaired loans	$ 6,199,000		$ 8,197,000		$ 6,199,000		$ 8,197,000		$ 6,270,000
TDRs	294,000	[1],[2]	5,245,000	[1],[2]	7,017,000	[1],[3]	25,648,000	[1],[3]
Weighted average extension term	17 months		13 months
Weighted average stated interest rate, basis points	1.80%		1.76%
Weighted average interest only term	24 months		9 months
Loan forgiveness	0		0
Modification to Interest Only Payments
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number	1	[1]	3	[1]
TDRs	50,000	[1]	2,439,000	[1]
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Restructured loans	67,218,000		92,517,000		67,218,000		92,517,000		82,275,000
TDRs, number	125
Allowance for loan losses related to impaired loans	866,000				866,000
Interest income, passage of time	193,000		132,000
Residential Real Estate [Member] | Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Repossessed Assets	$ 9,900,000				$ 9,900,000

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