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, 2018 USD ($) contracts	Sep. 30, 2017 USD ($) contracts	Sep. 30, 2018 USD ($) contracts	Sep. 30, 2017 USD ($) contracts	Sep. 30, 2018 USD ($) Loan	Sep. 30, 2017 USD ($) Loan
Financing Receivable, Modifications [Line Items]
TDRs, number	22	5	41	13	49	19
TDRs, Additions	$ 4,233	$ 1,663	$ 24,828	$ 5,386	$ 43,767	$ 8,834
Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	22	1	38	9
TDRs, Additions	$ 4,233	$ 186	$ 6,956	$ 3,909
Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	7	2	13	7
TDRs, Additions	$ 621	$ 255	$ 1,502	$ 2,589
Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	3	3	3
TDRs, Additions	$ 0	$ 1,408	$ 17,872	$ 1,408
Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	1	0	1
TDRs, Additions	$ 0	$ 69	$ 0	$ 69
Residential real estate and other
Financing Receivable, Modifications [Line Items]
TDRs, number	20	2	31	8	35	12
TDRs, Additions	$ 3,679	$ 255	$ 5,846	$ 2,638	$ 6,257	$ 3,137
Residential real estate and other | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	20	1	31	7
TDRs, Additions	$ 3,679	$ 186	$ 5,846	$ 2,569
Residential real estate and other | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	7	2	12	7
TDRs, Additions	$ 621	$ 255	$ 1,417	$ 2,589
Residential real estate and other | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Residential real estate and other | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	1	0	1
TDRs, Additions	$ 0	$ 69	$ 0	$ 69
Commercial, industrial and other | Commercial
Financing Receivable, Modifications [Line Items]
TDRs, number	1	3	4	4	5	4
TDRs, Additions	$ 519	$ 1,408	$ 13,442	$ 1,503	$ 15,714	$ 1,503
Commercial, industrial and other | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	0	3	1
TDRs, Additions	$ 519	$ 0	$ 692	$ 95
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	3	1	3
TDRs, Additions	$ 0	$ 1,408	$ 12,750	$ 1,408
Commercial, industrial and other | Commercial | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Franchise | Commercial
Financing Receivable, Modifications [Line Items]
TDRs, number	1	0	3	0	6	0
TDRs, Additions	$ 35	$ 0	$ 5,157	$ 0	$ 21,413	$ 0
Franchise | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	0	1	0
TDRs, Additions	$ 35	$ 0	$ 35	$ 0
Franchise | Commercial | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Franchise | Commercial | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	2	0
TDRs, Additions	$ 0	$ 0	$ 5,122	$ 0
Franchise | Commercial | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Leases | Commercial
Financing Receivable, Modifications [Line Items]
TDRs, number	0	0	1	0	1	2
TDRs, Additions	$ 0	$ 0	$ 239	$ 0	$ 239	$ 2,949
Leases | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	1	0
TDRs, Additions	$ 0	$ 0	$ 239	$ 0
Leases | Commercial | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Leases | Commercial | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Leases | Commercial | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Office | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	0	1	0	1	0
TDRs, Additions	$ 0	$ 0	$ 59	$ 0	$ 59	$ 0
Office | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	1	0
TDRs, Additions	$ 0	$ 0	$ 59	$ 0
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 | Loan					0
TDRs, Additions					$ 0
Mixed use and other | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number	0	0	1	1	1	1
TDRs, Additions	$ 0	$ 0	$ 85	$ 1,245	$ 85	$ 1,245
Mixed use and other | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	1	1
TDRs, Additions	$ 0	$ 0	$ 85	$ 1,245
Mixed use and other | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	1	0
TDRs, Additions	$ 0	$ 0	$ 85	$ 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