Allowance for Credit Losses (Summary of TDRs Subsequent Default Under the Restructured Terms) (Detail) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) contracts	Jun. 30, 2019 USD ($) contracts	Jun. 30, 2020 USD ($) contracts	Jun. 30, 2019 USD ($) contracts	Jun. 30, 2020 USD ($) contracts	Jun. 30, 2019 USD ($) contracts
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	29	30	67	60	183	112
TDRs, balance | $	$ 9,017	$ 9,489	$ 32,814	$ 33,207	$ 53,173	$ 41,854
Payments in default, count | contracts	25	17	27	18
Payments in default, balance | $	$ 12,940	$ 14,304	$ 14,012	$ 14,433
Commercial, industrial and other | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	7	5	12	14	22	17
TDRs, balance | $	$ 3,431	$ 2,455	$ 9,033	$ 21,385	$ 13,989	$ 22,067
Payments in default, count | contracts	7	7	8	7
Payments in default, balance | $	$ 4,252	$ 12,220	$ 5,013	$ 12,220
Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	1	3	14	4	17	5
TDRs, balance | $	$ 2,082	$ 1,273	$ 18,135	$ 1,575	$ 23,578	$ 1,945
Payments in default, count | contracts	6	3	6	3
Payments in default, balance | $	$ 6,181	$ 984	$ 6,181	$ 984
Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	21	22	41	42	144	90
TDRs, balance | $	$ 3,504	$ 5,761	$ 5,646	$ 10,247	$ 15,606	$ 17,842
Payments in default, count | contracts	12	7	13	8
Payments in default, balance | $	$ 2,507	$ 1,100	$ 2,818	$ 1,229
Extension at Below Market Terms
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	26	30	52	52
TDRs, balance | $	$ 3,948	$ 9,489	$ 23,665	$ 14,483
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	5	8	7
TDRs, balance | $	$ 443	$ 2,455	$ 4,759	$ 2,963
Extension at Below Market Terms | Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	0	3	11	3
TDRs, balance | $	$ 0	$ 1,273	$ 13,511	$ 1,273
Extension at Below Market Terms | Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	21	22	33	42
TDRs, balance | $	$ 3,505	$ 5,761	$ 5,395	$ 10,247
Reduction of Interest Rate
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	10	10	18	16
TDRs, balance | $	$ 1,590	$ 2,492	$ 3,297	$ 4,039
Reduction of Interest Rate | Commercial, industrial and other | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	0	1	0	1
TDRs, balance | $	$ 0	$ 550	$ 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	10	9	15	15
TDRs, balance | $	$ 1,590	$ 1,942	$ 2,376	$ 3,489
Modification to Interest Only Payments
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	5	2	10	10
TDRs, balance | $	$ 5,339	$ 1,494	$ 8,802	$ 20,218
Modification to Interest Only Payments | Commercial, industrial and other | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	4	2	4	9
TDRs, balance | $	$ 3,257	$ 1,494	$ 3,257	$ 19,916
Modification to Interest Only Payments | Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contracts	1	0	6	1
TDRs, balance | $	$ 2,082	$ 0	$ 5,545	$ 302
Modification to Interest Only Payments | 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
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