Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Summary of TDRs Subsequent Default Under the Restructured Terms) (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Loan contracts	Mar. 31, 2017 USD ($) Loan contracts	Mar. 31, 2018 USD ($) Loan	Mar. 31, 2017 USD ($) Loan
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	7	4	24	14
Total, Balance	$ 990	$ 1,513	$ 23,802	$ 5,687
Subsequent Default, Count | Loan	10	2
Subsequent Default, Balance	$ 6,327	$ 260
Commercial | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	1	1	5	3
Total, Balance	$ 96	$ 95	$ 3,776	$ 398
Subsequent Default, Count | Loan	5	1
Subsequent Default, Balance	$ 3,776	$ 28
Commercial | Leases
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count | Loan			3	2
Total, Balance			$ 16,256	$ 2,949
Subsequent Default, Count | Loan	0	0
Subsequent Default, Balance	$ 0	$ 0
Commercial real estate | Office
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	1	0	1	0
Total, Balance	$ 59	$ 0	$ 59	$ 0
Subsequent Default, Count | Loan	0	0
Subsequent Default, Balance	$ 0	$ 0
Commercial real estate | Industrial
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	0	0	0	0
Total, Balance	$ 0	$ 0	$ 0	$ 0
Subsequent Default, Count | Loan	0	0
Subsequent Default, Balance	$ 0	$ 0
Commercial real estate | Mixed use and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	0	1	0	1
Total, Balance	$ 0	$ 1,245	$ 0	$ 1,245
Subsequent Default, Count | Loan	0	0
Subsequent Default, Balance	$ 0	$ 0
Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	5	2	15	8
Total, Balance	$ 835	$ 173	$ 3,711	$ 1,095
Subsequent Default, Count | Loan	5	1
Subsequent Default, Balance	$ 2,551	$ 232