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		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) Loan contracts	Sep. 30, 2016 USD ($) Loan contracts	Sep. 30, 2017 USD ($) Loan contracts	Sep. 30, 2016 USD ($) Loan contracts	Sep. 30, 2017 USD ($) Loan	Sep. 30, 2016 USD ($) Loan
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	5	2	13	15	19	17
Total, Balance	$ 1,663	$ 71	$ 5,386	$ 9,449	$ 8,834	$ 9,650
Subsequent Default, Count | Loan	2	5	3	7
Subsequent Default, Balance	$ 1,297	$ 1,191	$ 1,529	$ 1,392
Commercial | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	3	1	4	3	4	3
Total, Balance	$ 1,408	$ 28	$ 1,503	$ 345	$ 1,503	$ 345
Subsequent Default, Count | Loan	0	0	0	0
Subsequent Default, Balance	$ 0	$ 0	$ 0	$ 0
Commercial | Leases
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count | Loan					2	0
Total, Balance					$ 2,949	$ 0
Subsequent Default, Count | Loan	0	0	0	0
Subsequent Default, Balance	$ 0	$ 0	$ 0	$ 0
Commercial real estate | Office
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	0	0	0	1	0	1
Total, Balance	$ 0	$ 0	$ 0	$ 450	$ 0	$ 450
Subsequent Default, Count | Loan	0	1	0	1
Subsequent Default, Balance	$ 0	$ 450	$ 0	$ 450
Commercial real estate | Industrial
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	0	0	0	6	0	6
Total, Balance	$ 0	$ 0	$ 0	$ 7,921	$ 0	$ 7,921
Subsequent Default, Count | Loan	0	3	0	3
Subsequent Default, Balance	$ 0	$ 725	$ 0	$ 725
Commercial real estate | Mixed use and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	0	0	1	2	1	4
Total, Balance	$ 0	$ 0	$ 1,245	$ 150	$ 1,245	$ 351
Subsequent Default, Count | Loan	1	1	1	3
Subsequent Default, Balance	$ 1,245	$ 16	$ 1,245	$ 217
Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	2	1	8	3	12	3
Total, Balance	$ 255	$ 43	$ 2,638	$ 583	$ 3,137	$ 583
Subsequent Default, Count | Loan	1	0	2	0
Subsequent Default, Balance	$ 52	$ 0	$ 284	$ 0