Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Summary of the Post-Modification Balance of TDRs) (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) contracts	Sep. 30, 2018 USD ($) contracts	Sep. 30, 2019 USD ($) contracts	Sep. 30, 2018 USD ($) contracts	Sep. 30, 2019 USD ($) contracts	Sep. 30, 2018 USD ($) contracts
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	68	22	127	41	157	49
TDRs, balance | $	$ 13,597,000	$ 4,233,000	$ 46,804,000	$ 24,828,000	$ 51,219,000	$ 43,767,000
Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	63	22	115	38
TDRs, balance | $	$ 12,481,000	$ 4,233,000	$ 26,964,000	$ 6,956,000
Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	9	7	25	13
TDRs, balance | $	$ 1,048,000	$ 621,000	$ 5,087,000	$ 1,502,000
Modification to Interest Only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	6	0	16	3
TDRs, balance | $	$ 6,309,000	$ 0	$ 26,527,000	$ 17,872,000
Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	0	0	0	0
TDRs, balance | $	$ 0	$ 0	$ 0	$ 0
Residential real estate and other
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	60	20	102	31	130	35
TDRs, balance | $	$ 4,879,000	$ 3,679,000	$ 15,126,000	$ 5,846,000	$ 19,041,000	$ 6,257,000
Residential real estate and other | Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	59	20	101	31
TDRs, balance | $	$ 4,568,000	$ 3,679,000	$ 14,815,000	$ 5,846,000
Residential real estate and other | Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	9	7	24	12
TDRs, balance | $	$ 1,048,000	$ 621,000	$ 4,537,000	$ 1,417,000
Residential real estate and other | Modification to Interest Only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	1	0	1	0
TDRs, balance | $	$ 311,000	$ 0	$ 311,000	$ 0
Residential real estate and other | Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	0	0	0	0
TDRs, balance | $	$ 0	$ 0	$ 0	$ 0
Commercial, industrial and other | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	5	1	18	4	18	5
TDRs, balance | $	$ 3,526,000	$ 519,000	$ 24,835,000	$ 13,442,000	$ 24,835,000	$ 15,714,000
Commercial, industrial and other | Commercial | Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	2	1	8	3
TDRs, balance | $	$ 2,721,000	$ 519,000	$ 5,608,000	$ 692,000
Commercial, industrial and other | Commercial | Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	0	0	1	0
TDRs, balance | $	$ 0	$ 0	$ 550,000	$ 0
Commercial, industrial and other | Commercial | Modification to Interest Only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	3	0	12	1
TDRs, balance | $	$ 807,000	$ 0	$ 20,723,000	$ 12,750,000
Commercial, industrial and other | Commercial | Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	0	0	0	0
TDRs, balance | $	$ 0	$ 0	$ 0	$ 0
Franchise | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts		1		3		6
TDRs, balance | $		$ 35,000		$ 5,157,000		$ 21,413,000
Franchise | Commercial | Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts		1		1
TDRs, balance | $		$ 35,000		$ 35,000
Franchise | Commercial | Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts		0		0
TDRs, balance | $		$ 0		$ 0
Franchise | Commercial | Modification to Interest Only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts		0		2
TDRs, balance | $		$ 0		$ 5,122,000
Franchise | Commercial | Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts		0		0
TDRs, balance | $		$ 0		$ 0
Asset-based lending | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts			1		2
TDRs, balance | $			$ 76,000		$ 206,000
Asset-based lending | Commercial | Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts			1
TDRs, balance | $			$ 76,000
Asset-based lending | Commercial | Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts			0
TDRs, balance | $			$ 0
Asset-based lending | Commercial | Modification to Interest Only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts			0
TDRs, balance | $			$ 0
Asset-based lending | Commercial | Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts			0
TDRs, balance | $			$ 0
Leases | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts		0		1		1
TDRs, balance | $		$ 0		$ 239,000		$ 239,000
Leases | Commercial | Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts		0		1
TDRs, balance | $		$ 0		$ 239,000
Leases | Commercial | Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts		0		0
TDRs, balance | $		$ 0		$ 0
Leases | Commercial | Modification to Interest Only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts		0		0
TDRs, balance | $		$ 0		$ 0
Leases | Commercial | Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts		0		0
TDRs, balance | $		$ 0		$ 0
Office | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	1		2	1	2	1
TDRs, balance | $	$ 5,070,000		$ 5,382,000	$ 59,000	$ 5,382,000	$ 59,000
Office | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	1		2	1
TDRs, balance | $	$ 5,070,000		$ 5,382,000	$ 59,000
Office | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	0		0	0
TDRs, balance | $	$ 0		$ 0	$ 0
Office | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	1		1	0
TDRs, balance | $	$ 5,070,000		$ 5,070,000	$ 0
Office | Commercial real estate | Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	0		0	0
TDRs, balance | $	$ 0		$ 0	$ 0
Mixed use and other | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	2	0	4	1	5	1
TDRs, balance | $	$ 122,000	$ 0	$ 1,385,000	$ 85,000	$ 1,755,000	$ 85,000
Mixed use and other | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	1	0	3	1
TDRs, balance | $	$ 122,000	$ 0	$ 1,083,000	$ 85,000
Mixed use and other | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	0	0	0	1
TDRs, balance | $	$ 0	$ 0	$ 0	$ 85,000
Mixed use and other | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	1	0	2	0
TDRs, balance | $	$ 121,000	$ 0	$ 423,000	$ 0
Mixed use and other | Commercial real estate | Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contracts	0	0	0	0
TDRs, balance | $	$ 0	$ 0	$ 0	$ 0