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, 2018 USD ($) contracts	Jun. 30, 2017 USD ($) contracts	Jun. 30, 2018 USD ($) contracts	Jun. 30, 2017 USD ($) contracts	Jun. 30, 2018 USD ($) Loan	Jun. 30, 2017 USD ($) Loan
Financing Receivable, Modifications [Line Items]
TDRs, number	12	4	19	8	32	16
TDRs, Additions	$ 19,605	$ 2,210	$ 20,595	$ 3,723	$ 41,197	$ 7,242
Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	9	4	16	8
TDRs, Additions	$ 1,733	$ 2,210	$ 2,723	$ 3,723
Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	4	3	6	5
TDRs, Additions	$ 770	$ 2,161	$ 881	$ 2,334
Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	3	0	3	0
TDRs, Additions	$ 17,872	$ 0	$ 17,872	$ 0
Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Residential real estate and other
Financing Receivable, Modifications [Line Items]
TDRs, number	6	4	11	6	17	11
TDRs, Additions	$ 1,332	$ 2,210	$ 2,167	$ 2,383	$ 2,833	$ 2,925
Residential real estate and other | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	6	4	11	6
TDRs, Additions	$ 1,332	$ 2,210	$ 2,167	$ 2,383
Residential real estate and other | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	3	3	5	5
TDRs, Additions	$ 685	$ 2,161	$ 796	$ 2,334
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	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 0
Commercial, industrial and other | Commercial
Financing Receivable, Modifications [Line Items]
TDRs, number	2	0	3	1	7	2
TDRs, Additions	$ 12,827	$ 0	$ 12,923	$ 95	$ 16,603	$ 123
Commercial, industrial and other | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	0	2	1
TDRs, Additions	$ 77	$ 0	$ 173	$ 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	1	0	1	0
TDRs, Additions	$ 12,750	$ 0	$ 12,750	$ 0
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	2	0	2	0	5	0
TDRs, Additions	$ 5,122	$ 0	$ 5,122	$ 0	$ 21,378	$ 0
Franchise | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0	0	0
TDRs, Additions	$ 0	$ 0	$ 0	$ 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	2	0	2	0
TDRs, Additions	$ 5,122	$ 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	1	0	1	0	1	2
TDRs, Additions	$ 239	$ 0	$ 239	$ 0	$ 239	$ 2,949
Leases | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	0	1	0
TDRs, Additions	$ 239	$ 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	1	0	1	1	1	1
TDRs, Additions	$ 85	$ 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	1	0	1	1
TDRs, Additions	$ 85	$ 0	$ 85	$ 1,245
Mixed use and other | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	0	1	0
TDRs, Additions	$ 85	$ 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