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		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) Loan contracts	Jun. 30, 2016 USD ($) Loan contracts	Jun. 30, 2017 USD ($) Loan contracts	Jun. 30, 2016 USD ($) Loan contracts	Jun. 30, 2017 USD ($) Loan	Jun. 30, 2016 USD ($) Loan
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	4	2	8	13	16	16
Total, Balance	$ 2,210	$ 655	$ 3,723	$ 9,378	$ 7,242	$ 9,801
Subsequent Default, Count | Loan	2	6	2	8
Subsequent Default, Balance	$ 260	$ 1,413	$ 260	$ 1,614
Commercial | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	0	1	1	2	2	2
Total, Balance	$ 0	$ 275	$ 95	$ 317	$ 123	$ 317
Subsequent Default, Count | Loan	1	0	1	0
Subsequent Default, Balance	$ 28	$ 0	$ 28	$ 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	0	1	0	3
Subsequent Default, Balance	$ 0	$ 16	$ 0	$ 217
Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	4	1	6	2	11	3
Total, Balance	$ 2,210	$ 380	$ 2,383	$ 540	$ 2,925	$ 762
Subsequent Default, Count | Loan	1	1	1	1
Subsequent Default, Balance	$ 232	$ 222	$ 232	$ 222