Allowance for Credit Losses (Summary of TDRs Subsequent Default Under the Restructured Terms) (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
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	9	36	45	103	66	152
TDRs, balance | $	$ 559	$ 7,596	$ 6,884	$ 40,410	$ 12,345	$ 47,173
Payments in default, count | contract	6	31	9	34
Payments in default, balance | $	$ 2,590	$ 21,333	$ 3,915	$ 21,784
Commercial, industrial and other | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	1	5	6	17	10	22
TDRs, balance | $	$ 1	$ 1,013	$ 547	$ 10,046	$ 2,863	$ 11,476
Payments in default, count | contract	1	7	4	7
Payments in default, balance | $	$ 199	$ 4,607	$ 1,524	$ 4,607
Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	0	3	5	17	6	18
TDRs, balance | $	$ 0	$ 1,045	$ 2,944	$ 19,180	$ 3,045	$ 19,432
Payments in default, count | contract	3	13	3	13
Payments in default, balance | $	$ 2,275	$ 14,595	$ 2,275	$ 14,595
Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	8	28	34	69	50	112
TDRs, balance | $	$ 558	$ 5,538	$ 3,393	$ 11,184	$ 6,437	$ 16,265
Payments in default, count | contract	2	11	2	14
Payments in default, balance | $	$ 116	$ 2,131	$ 116	$ 2,582