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, 2020 USD ($) contracts	Sep. 30, 2019 USD ($) contracts	Sep. 30, 2020 USD ($) contracts	Sep. 30, 2019 USD ($) contracts	Sep. 30, 2020 USD ($) contracts	Sep. 30, 2019 USD ($) contracts
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	36	68	103	127	152	157
TDRs, balance | $	$ 7,596	$ 13,597	$ 40,410	$ 46,804	$ 47,173	$ 51,219
Payments in default, count | contracts	31	18	34	23
Payments in default, balance | $	$ 21,333	$ 6,545	$ 21,784	$ 18,692
Commercial, industrial and other | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	5	5	17	19	22	20
TDRs, balance | $	$ 1,013	$ 3,526	$ 10,046	$ 24,911	$ 11,476	$ 25,041
Payments in default, count | contracts	7	5	7	8
Payments in default, balance | $	$ 4,607	$ 1,949	$ 4,607	$ 13,554
Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	3	3	17	6	18	7
TDRs, balance | $	$ 1,045	$ 5,192	$ 19,180	$ 6,767	$ 19,432	$ 7,137
Payments in default, count | contracts	13	2	13	3
Payments in default, balance | $	$ 14,595	$ 682	$ 14,595	$ 984
Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	28	60	69	102	112	130
TDRs, balance | $	$ 5,538	$ 4,879	$ 11,184	$ 15,126	$ 16,265	$ 19,041
Payments in default, count | contracts	11	11	14	12
Payments in default, balance | $	$ 2,131	$ 3,914	$ 2,582	$ 4,154
Extension at Below Market Terms
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	29	63	81	115
TDRs, balance | $	$ 7,339	$ 12,481	$ 31,004	$ 26,964
Extension at Below Market Terms | Commercial, industrial and other | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	5	2	13	9
TDRs, balance | $	$ 1,013	$ 2,721	$ 5,772	$ 5,684
Extension at Below Market Terms | Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	3	2	14	5
TDRs, balance | $	$ 1,045	$ 5,192	$ 14,556	$ 6,465
Extension at Below Market Terms | Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	21	59	54	101
TDRs, balance | $	$ 5,281	$ 4,568	$ 10,676	$ 14,815
Reduction of Interest Rate
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	13	9	31	25
TDRs, balance | $	$ 1,829	$ 1,048	$ 5,126	$ 5,087
Reduction of Interest Rate | Commercial, industrial and other | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	0	0	0	1
TDRs, balance | $	$ 0	$ 0	$ 0	$ 550
Reduction of Interest Rate | Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	0	0	3	0
TDRs, balance | $	$ 0	$ 0	$ 921	$ 0
Reduction of Interest Rate | Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	13	9	28	24
TDRs, balance | $	$ 1,829	$ 1,048	$ 4,205	$ 4,537
Modification to Interest Only Payments
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	2	6	12	16
TDRs, balance | $	$ 397	$ 6,309	$ 9,199	$ 26,527
Modification to Interest Only Payments | Commercial, industrial and other | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	0	3	4	12
TDRs, balance | $	$ 0	$ 807	$ 3,257	$ 20,723
Modification to Interest Only Payments | Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	1	2	7	3
TDRs, balance | $	$ 207	$ 5,191	$ 5,752	$ 5,493
Modification to Interest Only Payments | Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	1	1	1	1
TDRs, balance | $	$ 190	$ 311	$ 190	$ 311
Forgiveness of Debt
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	0	0	1	0
TDRs, balance | $	$ 0	$ 0	$ 432	$ 0
Forgiveness of Debt | Commercial, industrial and other | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	0	0	1	0
TDRs, balance | $	$ 0	$ 0	$ 432	$ 0
Forgiveness of Debt | Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	0	0	0	0
TDRs, balance | $	$ 0	$ 0	$ 0	$ 0
Forgiveness of Debt | Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	0	0	0	0
TDRs, balance | $	$ 0	$ 0	$ 0	$ 0