Allowance for Credit Losses (Summary of TDRs Subsequent Default Under the Restructured Terms) (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) contract	Mar. 31, 2020 USD ($) contract	Mar. 31, 2021 USD ($) contract	Mar. 31, 2020 USD ($) contract
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	20	38	106	184
TDRs, balance | $	$ 2,126	$ 23,797	$ 24,201	$ 53,646
Payments in default, count | contract	13	22
Payments in default, balance | $	$ 5,213	$ 8,104
Commercial, industrial and other, excluding PPP loans | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	2	5	18	20
TDRs, balance | $	$ 151	$ 5,602	$ 6,910	$ 13,013
Payments in default, count | contract	5	8
Payments in default, balance | $	$ 1,496	$ 4,836
Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	2	13	7	19
TDRs, balance | $	$ 237	$ 16,053	$ 3,466	$ 22,770
Payments in default, count | contract	3	3
Payments in default, balance | $	$ 3,121	$ 758
Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	16	20	81	145
TDRs, balance | $	$ 1,738	$ 2,142	$ 13,825	$ 17,863
Payments in default, count | contract	5	11
Payments in default, balance | $	$ 596	$ 2,510
Extension at Below Market Terms
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	20	26
TDRs, balance | $	$ 2,126	$ 19,717
Extension at Below Market Terms | Commercial, industrial and other, excluding PPP loans | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	2	3
TDRs, balance | $	$ 151	$ 4,316
Extension at Below Market Terms | Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	2	11
TDRs, balance | $	$ 237	$ 13,511
Extension at Below Market Terms | Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	16	12
TDRs, balance | $	$ 1,738	$ 1,890
Reduction of Interest Rate
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	10	8
TDRs, balance | $	$ 1,403	$ 1,707
Reduction of Interest Rate | Commercial, industrial and other, excluding PPP loans | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	0	0
TDRs, balance | $	$ 0	$ 0
Reduction of Interest Rate | Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	1	3
TDRs, balance | $	$ 113	$ 921
Reduction of Interest Rate | Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	9	5
TDRs, balance | $	$ 1,290	$ 786
Modification to Interest Only Payments
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	1	5
TDRs, balance | $	$ 113	$ 3,463
Modification to Interest Only Payments | Commercial, industrial and other, excluding PPP loans | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	0	0
TDRs, balance | $	$ 0	$ 0
Modification to Interest Only Payments | Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	1	5
TDRs, balance | $	$ 113	$ 3,463
Modification to Interest Only Payments | Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	0	0
TDRs, balance | $	$ 0	$ 0
Forgiveness of Debt
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	0	1
TDRs, balance | $	$ 0	$ 432
Forgiveness of Debt | Commercial, industrial and other, excluding PPP loans | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	0	1
TDRs, balance | $	$ 0	$ 432
Forgiveness of Debt | Commercial real estate | Non-construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	0	0
TDRs, balance | $	$ 0	$ 0
Forgiveness of Debt | Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
TDRs, count | contract	0	0
TDRs, balance | $	$ 0	$ 0