Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans 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, 2015 USD ($) contracts		Sep. 30, 2014 USD ($) contracts		Sep. 30, 2015 USD ($) contracts		Sep. 30, 2014 USD ($) contracts		Sep. 30, 2015 USD ($) contracts		Sep. 30, 2014 USD ($) contracts
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count		1	[1],[2]	3	[1],[2]	10	[1],[2]	16	[1],[2]	15	[3],[4]	21	[3],[4]
Total, Balance | $		$ 222	[1],[2]	$ 667	[1],[2]	$ 1,833	[1],[2]	$ 8,152	[1],[2]	$ 5,648	[3],[4]	$ 10,086	[3],[4]
Subsequent Default, Count	[3],[5]	3		6		4		6
Subsequent Default, Balance | $	[3],[5]	$ 816		$ 2,028		$ 1,030		$ 2,028
Commercial | Commercial and industrial
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count		0	[1],[2]	0	[1],[2]	0	[1],[2]	1	[1],[2]	1	[3],[4]	1	[3],[4]
Total, Balance | $		$ 0	[1],[2]	$ 0	[1],[2]	$ 0	[1],[2]	$ 88	[1],[2]	$ 1,461	[3],[4]	$ 88	[3],[4]
Subsequent Default, Count	[3],[5]	0		1		0		1
Subsequent Default, Balance | $	[3],[5]	$ 0		$ 88		$ 0		$ 88
Commercial real estate | Land
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count	[3],[4]									0		0
Total, Balance | $	[3],[4]									$ 0		$ 0
Subsequent Default, Count	[3],[5]	0		0		0		0
Subsequent Default, Balance | $	[3],[5]	$ 0		$ 0		$ 0		$ 0
Commercial real estate | Office
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count		0	[1],[2]	0	[1],[2]	0	[1],[2]	1	[1],[2]	1	[3],[4]	1	[3],[4]
Total, Balance | $		$ 0	[1],[2]	$ 0	[1],[2]	$ 0	[1],[2]	$ 790	[1],[2]	$ 720	[3],[4]	$ 790	[3],[4]
Subsequent Default, Count	[3],[5]	0		0		0		0
Subsequent Default, Balance | $	[3],[5]	$ 0		$ 0		$ 0		$ 0
Commercial real estate | Industrial
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count		0	[1],[2]	0	[1],[2]	1	[1],[2]	1	[1],[2]	2	[3],[4]	1	[3],[4]
Total, Balance | $		$ 0	[1],[2]	$ 0	[1],[2]	$ 169	[1],[2]	$ 1,078	[1],[2]	$ 854	[3],[4]	$ 1,078	[3],[4]
Subsequent Default, Count	[3],[5]	1		1		1		1
Subsequent Default, Balance | $	[3],[5]	$ 685		$ 1,078		$ 685		$ 1,078
Commercial real estate | Retail
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count		0	[1],[2]	0	[1],[2]	0	[1],[2]	1	[1],[2]	0	[3],[4]	1	[3],[4]
Total, Balance | $		$ 0	[1],[2]	$ 0	[1],[2]	$ 0	[1],[2]	$ 202	[1],[2]	$ 0	[3],[4]	$ 202	[3],[4]
Subsequent Default, Count	[3],[5]	0		0		0		0
Subsequent Default, Balance | $	[3],[5]	$ 0		$ 0		$ 0		$ 0
Commercial real estate | Multi-family
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count		0	[1],[2]	0	[1],[2]	0	[1],[2]	1	[1],[2]	0	[3],[4]	1	[3],[4]
Total, Balance | $		$ 0	[1],[2]	$ 0	[1],[2]	$ 0	[1],[2]	$ 181	[1],[2]	$ 0	[3],[4]	$ 181	[3],[4]
Subsequent Default, Count	[3],[5]	0		0		0		0
Subsequent Default, Balance | $	[3],[5]	$ 0		$ 0		$ 0		$ 0
Commercial real estate | Mixed use and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count		0	[1],[2]	0	[1],[2]	0	[1],[2]	7	[1],[2]	0	[3],[4]	10	[3],[4]
Total, Balance | $		$ 0	[1],[2]	$ 0	[1],[2]	$ 0	[1],[2]	$ 4,926	[1],[2]	$ 0	[3],[4]	$ 6,341	[3],[4]
Subsequent Default, Count	[3],[5]	0		2		0		2
Subsequent Default, Balance | $	[3],[5]	$ 0		$ 482		$ 0		$ 482
Residential real estate and other
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Total, Count		1	[1],[2]	3	[1],[2]	9	[1],[2]	4	[1],[2]	11	[3],[4]	6	[3],[4]
Total, Balance | $		$ 222	[1],[2]	$ 667	[1],[2]	$ 1,664	[1],[2]	$ 887	[1],[2]	$ 2,613	[3],[4]	$ 1,406	[3],[4]
Subsequent Default, Count	[3],[5]	2		2		3		2
Subsequent Default, Balance | $	[3],[5]	$ 131		$ 380		$ 345		$ 380

[1] Balances represent the recorded investment in the loan at the time of the restructuring.

[2] TDRs may have more than one modification representing a concession. As such, TDRs during the period may be represented in more than one of the categories noted above.

[3] Balances represent the recorded investment in the loan at the time of the restructuring.

[4] Total TDRs represent all loans restructured in TDRs during the previous twelve months from the date indicated.

[5] TDRs considered to be in payment default are over 30 days past-due subsequent to the restructuring.