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, 2016 USD ($) contracts	Sep. 30, 2015 USD ($) contracts	Sep. 30, 2016 USD ($) contracts	Sep. 30, 2015 USD ($) contracts	Sep. 30, 2016 USD ($) Loan	Sep. 30, 2015 USD ($) Loan
Financing Receivable, Modifications [Line Items]
TDRs, number	2	1	15	10	17	15
TDRs, Additions	$ 71	$ 222	$ 9,449	$ 1,833	$ 9,650	$ 5,648
Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	2	1	14	10
TDRs, Additions	$ 71	$ 222	$ 9,289	$ 1,833
Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	1	6	5
TDRs, Additions	$ 43	$ 222	$ 7,779	$ 674
Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	1	2
TDRs, Additions	$ 0	$ 0	$ 380	$ 219
Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	1	0
TDRs, Additions	$ 0	$ 0	$ 275	$ 0
Residential real estate and other
Financing Receivable, Modifications [Line Items]
TDRs, number	1	1	3	9	3	11
TDRs, Additions	$ 43	$ 222	$ 583	$ 1,664	$ 583	$ 2,613
Residential real estate and other | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	1	2	9
TDRs, Additions	$ 43	$ 222	$ 423	$ 1,664
Residential real estate and other | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	1	3	5
TDRs, Additions	$ 43	$ 222	$ 583	$ 674
Residential real estate and other | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	1	1
TDRs, Additions	$ 0	$ 0	$ 380	$ 50
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	1	0	3	0	3	1
TDRs, Additions	$ 28	$ 0	$ 345	$ 0	$ 345	$ 1,461
Commercial, industrial and other | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	0	3	0
TDRs, Additions	$ 28	$ 0	$ 345	$ 0
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	0	1	0
TDRs, Additions	$ 0	$ 0	$ 275	$ 0
Office | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	0	1		1	1
TDRs, Additions	$ 0	$ 0	$ 450		$ 450	$ 720
Office | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	1
TDRs, Additions	$ 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
TDRs, Additions	$ 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	6	1	6	2
TDRs, Additions	$ 0	$ 0	$ 7,921	$ 169	$ 7,921	$ 854
Industrial | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	6	1
TDRs, Additions	$ 0	$ 0	$ 7,921	$ 169
Industrial | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	3	0
TDRs, Additions	$ 0	$ 0	$ 7,196	$ 0
Industrial | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	1
TDRs, Additions	$ 0	$ 0	$ 0	$ 169
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	2	0	4	0
TDRs, Additions	$ 0	$ 0	$ 150	$ 0	$ 351	$ 0
Mixed use and other | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	2	0
TDRs, Additions	$ 0	$ 0	$ 150	$ 0
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