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 (Summary of the Post-Modification Balance of TDRs) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2015 contracts		Mar. 31, 2014 contracts		Mar. 31, 2015 contracts		Mar. 31, 2014 contracts
Financing Receivable, Modifications [Line Items]
TDRs, number	3	[1],[2]	6	[1],[2]	18	[1],[3]	27	[1],[3]
TDRs	$ 294	[1],[2]	$ 5,245	[1],[2]	$ 7,017	[1],[3]	$ 25,648	[1],[3]
Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number	3	[1]	5	[1]
TDRs	294	[1]	3,972	[1]
Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number	2	[1]	3	[1]
TDRs	80	[1]	3,877	[1]
Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number	1	[1]	3	[1]
TDRs	50	[1]	2,439	[1]
Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	0	[1]
TDRs	0	[1]	0	[1]
Residential real estate and other
Financing Receivable, Modifications [Line Items]
TDRs, number	3	[1],[2]	0	[1],[2]	9	[1],[3]	6	[1],[3]
TDRs	294	[1],[2]	0	[1],[2]	2,131	[1],[3]	1,919	[1],[3]
Residential real estate and other | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number	3	[1]	0	[1]
TDRs	294	[1]	0	[1]
Residential real estate and other | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number	2	[1]	0	[1]
TDRs	80	[1]	0	[1]
Residential real estate and other | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number	1	[1]	0	[1]
TDRs	50	[1]	0	[1]
Residential real estate and other | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	0	[1]
TDRs	0	[1]	0	[1]
Commercial and industrial | Commercial
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1],[2]	1	[1],[2]	1	[1],[3]	1	[1],[3]
TDRs	0	[1],[2]	88	[1],[2]	1,461	[1],[3]	88	[1],[3]
Commercial and industrial | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	1	[1]
TDRs	0	[1]	88	[1]
Commercial and industrial | Commercial | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	0	[1]
TDRs	0	[1]	0	[1]
Commercial and industrial | Commercial | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	1	[1]
TDRs	0	[1]	88	[1]
Commercial and industrial | Commercial | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	0	[1]
TDRs	0	[1]	0	[1]
Industrial | Commercial real-estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1],[2]	1	[1],[2]	1	[1],[3]	2	[1],[3]
TDRs	0	[1],[2]	1,078	[1],[2]	685	[1],[3]	2,027	[1],[3]
Industrial | Commercial real-estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	1	[1]
TDRs	0	[1]	1,078	[1]
Industrial | Commercial real-estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	0	[1]
TDRs	0	[1]	0	[1]
Industrial | Commercial real-estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	1	[1]
TDRs	0	[1]	1,078	[1]
Industrial | Commercial real-estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	0	[1]
TDRs	0	[1]	0	[1]
Retail | Commercial real-estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1],[2]	1	[1],[2]	0	[1],[3]	1	[1],[3]
TDRs	0	[1],[2]	202	[1],[2]	0	[1],[3]	202	[1],[3]
Retail | Commercial real-estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	1	[1]
TDRs	0	[1]	202	[1]
Retail | Commercial real-estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	0	[1]
TDRs	0	[1]	0	[1]
Retail | Commercial real-estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	0	[1]
TDRs	0	[1]	0	[1]
Retail | Commercial real-estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	0	[1]
TDRs	0	[1]	0	[1]
Mixed use and other | Commercial real-estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1],[2]	3	[1],[2]	4	[1],[3]	9	[1],[3]
TDRs	0	[1],[2]	3,877	[1],[2]	1,049	[1],[3]	8,919	[1],[3]
Mixed use and other | Commercial real-estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	2	[1]
TDRs	0	[1]	2,604	[1]
Mixed use and other | Commercial real-estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	3	[1]
TDRs	0	[1]	3,877	[1]
Mixed use and other | Commercial real-estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	1	[1]
TDRs	0	[1]	1,273	[1]
Mixed use and other | Commercial real-estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number	0	[1]	0	[1]
TDRs	$ 0	[1]	$ 0	[1]

[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.