Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Summary of the Post-Modification Balance of TDRs) (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) contracts	Mar. 31, 2017 USD ($) contracts	Mar. 31, 2018 USD ($) Loan	Mar. 31, 2017 USD ($) Loan
Financing Receivable, Modifications [Line Items]
TDRs, number	7	4	24	14
TDRs, Additions	$ 990	$ 1,513	$ 23,802	$ 5,687
Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	7	4
TDRs, Additions	$ 990	$ 1,513
Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	2	2
TDRs, Additions	$ 111	$ 173
Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Residential real estate and other
Financing Receivable, Modifications [Line Items]
TDRs, number	5	2	15	8
TDRs, Additions	$ 835	$ 173	$ 3,711	$ 1,095
Residential real estate and other | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	5	2
TDRs, Additions	$ 835	$ 173
Residential real estate and other | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	2	2
TDRs, Additions	$ 111	$ 173
Residential real estate and other | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Residential real estate and other | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Commercial, industrial and other | Commercial
Financing Receivable, Modifications [Line Items]
TDRs, number	1	1	5	3
TDRs, Additions	$ 96	$ 95	$ 3,776	$ 398
Commercial, industrial and other | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	1
TDRs, Additions	$ 96	$ 95
Commercial, industrial and other | Commercial | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Commercial, industrial and other | Commercial | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Commercial, industrial and other | Commercial | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Office | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number	1	0	1	0
TDRs, Additions	$ 59	$ 0	$ 59	$ 0
Office | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	0
TDRs, Additions	$ 59	$ 0
Office | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Office | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Office | Commercial real estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Industrial | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Industrial | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Industrial | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Industrial | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Industrial | Commercial real estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Mixed use and other | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	1	0	1
TDRs, Additions	$ 0	$ 1,245	$ 0	$ 1,245
Mixed use and other | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	1
TDRs, Additions	$ 0	$ 1,245
Mixed use and other | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Mixed use and other | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Mixed use and other | Commercial real estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0