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, 2018 USD ($) Loan contracts	Jun. 30, 2017 USD ($) Loan contracts	Jun. 30, 2018 USD ($) Loan contracts	Jun. 30, 2017 USD ($) Loan contracts	Jun. 30, 2018 USD ($) Loan	Jun. 30, 2017 USD ($) Loan
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	12	4	19	8	32	16
Total, Balance	$ 19,605	$ 2,210	$ 20,595	$ 3,723	$ 41,197	$ 7,242
Subsequent Default, Count | Loan	8	2	8	2
Subsequent Default, Balance	$ 4,331	$ 260	$ 4,331	$ 260
Commercial | Commercial, industrial and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	2	0	3	1	7	2
Total, Balance	$ 12,827	$ 0	$ 12,923	$ 95	$ 16,603	$ 123
Subsequent Default, Count | Loan	5	1	5	1
Subsequent Default, Balance	$ 3,776	$ 28	$ 3,776	$ 28
Commercial | Franchise
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	2	0	2	0	5	0
Total, Balance	$ 5,122	$ 0	$ 5,122	$ 0	$ 21,378	$ 0
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	1	0	1	0	1	2
Total, Balance	$ 239	$ 0	$ 239	$ 0	$ 239	$ 2,949
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	1	0	1	0
Total, Balance	$ 0	$ 0	$ 59	$ 0	$ 59	$ 0
Subsequent Default, Count | Loan	0	0	0	0
Subsequent Default, Balance	$ 0	$ 0	$ 0	$ 0
Commercial real estate | Industrial
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count | Loan					0
Total, Balance					$ 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	1	0	1	1	1	1
Total, Balance	$ 85	$ 0	$ 85	$ 1,245	$ 85	$ 1,245
Subsequent Default, Count | Loan	0	0	0	0
Subsequent Default, Balance	$ 0	$ 0	$ 0	$ 0
Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	6	4	11	6	17	11
Total, Balance	$ 1,332	$ 2,210	$ 2,167	$ 2,383	$ 2,833	$ 2,925
Subsequent Default, Count | Loan	3	1	3	1
Subsequent Default, Balance	$ 555	$ 232	$ 555	$ 232