Allowance for Credit Losses (Summary of the Post-Modification Balance of TDRs) (Detail) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	9	36	45	103	66	152
TDRs, balance | $	$ 559	$ 7,596	$ 6,884	$ 40,410	$ 12,345	$ 47,173
Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	8	29	43	81
TDRs, balance | $	$ 442	$ 7,339	$ 6,224	$ 31,004
Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	2	13	15	31
TDRs, balance | $	$ 202	$ 1,829	$ 2,764	$ 5,126
Modification to Interest-only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	2	1	12
TDRs, balance | $	$ 0	$ 397	$ 113	$ 9,199
Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	0	0	1
TDRs, balance | $	$ 0	$ 0	$ 0	$ 432
Commercial, industrial and other | Commercial, industrial and other
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	1	5	6	17	10	22
TDRs, balance | $	$ 1	$ 1,013	$ 547	$ 10,046	$ 2,863	$ 11,476
Commercial, industrial and other | Commercial, industrial and other | Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	1	5	6	13
TDRs, balance | $	$ 1	$ 1,013	$ 547	$ 5,772
Commercial, industrial and other | Commercial, industrial and other | Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	0	0	0
TDRs, balance | $	$ 0	$ 0	$ 0	$ 0
Commercial, industrial and other | Commercial, industrial and other | Modification to Interest-only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	0	0	4
TDRs, balance | $	$ 0	$ 0	$ 0	$ 3,257
Commercial, industrial and other | Commercial, industrial and other | Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	0	0	1
TDRs, balance | $	$ 0	$ 0	$ 0	$ 432
Commercial real estate | Non-construction
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	3	5	17	6	18
TDRs, balance | $	$ 0	$ 1,045	$ 2,944	$ 19,180	$ 3,045	$ 19,432
Commercial real estate | Non-construction | Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	3	4	14
TDRs, balance | $	$ 0	$ 1,045	$ 2,401	$ 14,556
Commercial real estate | Non-construction | Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	0	2	3
TDRs, balance | $	$ 0	$ 0	$ 656	$ 921
Commercial real estate | Non-construction | Modification to Interest-only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	1	1	7
TDRs, balance | $	$ 0	$ 207	$ 113	$ 5,752
Commercial real estate | Non-construction | Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	0	0	0
TDRs, balance | $	$ 0	$ 0	$ 0	$ 0
Residential real estate and other
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	8	28	34	69	50	112
TDRs, balance | $	$ 558	$ 5,538	$ 3,393	$ 11,184	$ 6,437	$ 16,265
Residential real estate and other | Extension at Below Market Terms
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	7	21	33	54
TDRs, balance | $	$ 441	$ 5,281	$ 3,276	$ 10,676
Residential real estate and other | Reduction of Interest Rate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	2	13	13	28
TDRs, balance | $	$ 202	$ 1,829	$ 2,108	$ 4,205
Residential real estate and other | Modification to Interest-only Payments
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	1	0	1
TDRs, balance | $	$ 0	$ 190	$ 0	$ 190
Residential real estate and other | Forgiveness of Debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDRs, count | contract	0	0	0	0
TDRs, balance | $	$ 0	$ 0	$ 0	$ 0