Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Narrative) (Detail)		3 Months Ended				12 Months Ended
		Mar. 31, 2016 USD ($) contracts Rate		Mar. 31, 2015 USD ($) contracts Rate		Mar. 31, 2016 USD ($) Loan		Mar. 31, 2015 USD ($) Loan		Dec. 31, 2015 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number		11	[1],[2]	3	[1],[2]	20	[3],[4]	18	[3],[4]
Allowance for loan losses related to impaired loans		$ 7,775,000		$ 6,199,000		$ 7,775,000		$ 6,199,000		$ 6,380,000
TDRs, Additions		$ 8,723,000	[1],[2]	$ 294,000	[1],[2]	10,463,000	[3],[4]	7,017,000	[3],[4]
Weighted average extension term		3 months		17 months
Weighted average stated interest rate, basis points | Rate		0.17%		1.80%
Weighted average interest only term				24 months
Loan forgiveness		$ 0		$ 0
Modification to Interest Only Payments
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number | contracts	[1]	0		1
TDRs, Additions	[1]	$ 0		$ 50,000
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs		$ 52,555,000		67,218,000		52,555,000		$ 67,218,000		$ 51,853,000
TDRs, number | contracts		102
Allowance for loan losses related to impaired loans		$ 3,100,000				3,100,000
Interest income, passage of time		90,000		$ 193,000
Residential Real Estate | Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Foreclosed residential properties		$ 13,300,000				$ 13,300,000

[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.