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		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) contracts	Jun. 30, 2016 USD ($) contracts	Jun. 30, 2017 USD ($) contracts	Jun. 30, 2016 USD ($) contracts	Jun. 30, 2017 USD ($) Loan	Jun. 30, 2016 USD ($) Loan
Financing Receivable, Modifications [Line Items]
TDRs, number	4	2	8	13	16	16
TDRs, Additions	$ 2,210	$ 655	$ 3,723	$ 9,378	$ 7,242	$ 9,801
Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	4	2	8	12
TDRs, Additions	$ 2,210	$ 655	$ 3,723	$ 9,218
Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	3	1	5	5
TDRs, Additions	$ 2,161	$ 380	$ 2,334	$ 7,736
Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	1	0	1
TDRs, Additions	$ 0	$ 380	$ 0	$ 380
Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	1	0	1
TDRs, Additions	$ 0	$ 275	$ 0	$ 275
Residential real estate and other
Financing Receivable, Modifications [Line Items]
TDRs, number	4	1	6	2	11	3
TDRs, Additions	$ 2,210	$ 380	$ 2,383	$ 540	$ 2,925	$ 762
Residential real estate and other | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	4	1	6	1
TDRs, Additions	$ 2,210	$ 380	$ 2,383	$ 380
Residential real estate and other | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	3	1	5	2
TDRs, Additions	$ 2,161	$ 380	$ 2,334	$ 540
Residential real estate and other | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	1	0	1
TDRs, Additions	$ 0	$ 380	$ 0	$ 380
Residential real estate and other | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Commercial, industrial and other | Commercial
Financing Receivable, Modifications [Line Items]
TDRs, number	0	1	1	2	2	2
TDRs, Additions	$ 0	$ 275	$ 95	$ 317	$ 123	$ 317
Commercial, industrial and other | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	1	1	2
TDRs, Additions	$ 0	$ 275	$ 95	$ 317
Commercial, industrial and other | Commercial | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Commercial, industrial and other | Commercial | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Commercial, industrial and other | Commercial | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	1	0	1
TDRs, Additions	$ 0	$ 275	$ 0	$ 275
Office | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	0	0	1	0	1
TDRs, Additions	$ 0	$ 0	$ 0	$ 450	$ 0	$ 450
Office | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	1
TDRs, Additions	$ 0	$ 0	$ 0	$ 450
Office | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Office | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Office | Commercial real estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Industrial | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	0	0	6	0	6
TDRs, Additions	$ 0	$ 0	$ 0	$ 7,921	$ 0	$ 7,921
Industrial | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	6
TDRs, Additions	$ 0	$ 0	$ 0	$ 7,921
Industrial | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	3
TDRs, Additions	$ 0	$ 0	$ 0	$ 7,196
Industrial | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Industrial | Commercial real estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Mixed use and other | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	0	1	2	1	4
TDRs, Additions	$ 0	$ 0	$ 1,245	$ 150	$ 1,245	$ 351
Mixed use and other | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	1	2
TDRs, Additions	$ 0	$ 0	$ 1,245	$ 150
Mixed use and other | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Mixed use and other | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Mixed use and other | Commercial real estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0