Loans and Other Real Estate - Narrative (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) security loan	Sep. 30, 2019 security	Sep. 30, 2020 USD ($) loan security	Sep. 30, 2019 USD ($) security	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Overdraft balances included in home equity and consumer loans	$ 254,000		$ 254,000			$ 789,000
Residential loans pledged for potential borrowings at the Federal Home Loan Bank of New York	2,150,000,000		2,150,000,000			1,290,000,000
Troubled debt restructurings included in non accrual loans	1,300,000		1,300,000			1,600,000
Interest income on credit impaired loans			288,000	$ 342,000
Interest accrued on impaired loans			1,300,000	$ 703,000
Off-balance sheet, credit loss, liability	$ 2,000,000.0		$ 2,000,000.0			1,800,000
Number of Contracts | security	0	2	0	2
Other real estate owned	$ 0		$ 0			563,000
Other repossessed assets owned	$ 0		$ 0			0
Paycheck Protection Program ("PPP")
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan guarantee percentage	100.00%		100.00%
Home equity and consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts | security	0	2	0	2
Residential Mortgages And Consumer Home Equity Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans that were in process of foreclosure	$ 1,800,000		$ 1,800,000			$ 2,000,000.0
COVID-19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred payments for loans	$ 6,400,000		$ 6,400,000
Loans with payment deferments, number | loan	6		6
COVID-19 | Total Commercial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred payments for loans	$ 149,000,000.0		$ 149,000,000.0
COVID-19 | Total Commercial Loans | Payment Deferral
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, non-TDR modifications					$ 927,000,000.0
Deferred payment, period			90 days
Loans with deferred payments, percentage	91.00%		91.00%
COVID-19 | Equipment finance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred payments for loans	$ 3,000,000.0		$ 3,000,000.0		40,000,000.0
COVID-19 | Real estate - residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred payments for loans					$ 53,000,000.0
COVID-19 | Home equity and consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred payments for loans	3,000,000.0		3,000,000.0
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Group of impaired loans with recorded investment	$ 500,000		$ 500,000