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		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) contracts	Sep. 30, 2016 USD ($) contracts	Sep. 30, 2017 USD ($) contracts	Sep. 30, 2016 USD ($) contracts	Sep. 30, 2017 USD ($) Loan	Sep. 30, 2016 USD ($) Loan
Financing Receivable, Modifications [Line Items]
TDRs, number	5	2	13	15	19	17
TDRs, Additions	$ 1,663	$ 71	$ 5,386	$ 9,449	$ 8,834	$ 9,650
Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	2	9	14
TDRs, Additions	$ 186	$ 71	$ 3,909	$ 9,289
Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	2	1	7	6
TDRs, Additions	$ 255	$ 43	$ 2,589	$ 7,779
Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	3	0	3	1
TDRs, Additions	$ 1,408	$ 0	$ 1,408	$ 380
Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	0	1	1
TDRs, Additions	$ 69	$ 0	$ 69	$ 275
Residential real estate and other
Financing Receivable, Modifications [Line Items]
TDRs, number	2	1	8	3	12	3
TDRs, Additions	$ 255	$ 43	$ 2,638	$ 583	$ 3,137	$ 583
Residential real estate and other | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	1	7	2
TDRs, Additions	$ 186	$ 43	$ 2,569	$ 423
Residential real estate and other | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	2	1	7	3
TDRs, Additions	$ 255	$ 43	$ 2,589	$ 583
Residential real estate and other | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	1
TDRs, Additions	$ 0	$ 0	$ 0	$ 380
Residential real estate and other | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	0	1	0
TDRs, Additions	$ 69	$ 0	$ 69	$ 0
Commercial, industrial and other | Commercial
Financing Receivable, Modifications [Line Items]
TDRs, number	3	1	4	3	4	3
TDRs, Additions	$ 1,408	$ 28	$ 1,503	$ 345	$ 1,503	$ 345
Commercial, industrial and other | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	1	1	3
TDRs, Additions	$ 0	$ 28	$ 95	$ 345
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	3	0	3	0
TDRs, Additions	$ 1,408	$ 0	$ 1,408	$ 0
Commercial, industrial and other | Commercial | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	1
TDRs, Additions	$ 0	$ 0	$ 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