Allowance for Loan Losses and Credit Quality Information - Additional Information (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs		$ 14,200	$ 10,100
Percentage of average loans annualized, charged-offs		0.29%	0.30%
Accrued troubled debt restructured loans	$ 15,000	$ 15,000	$ 20,100	$ 14,300
Acquired nonimpaired loans	429,982	429,982	565,516	724,063
Reverse mortgage, fair value	16,500	16,500	19,800
Acquired impaired loans	4,300	4,300	5,800
Contractual principal balance	4,800	4,800	6,800
Interest income on impaired loans		800	1,000
Troubled debt restructuring related reserves	1,200	1,200	1,000
Private Banking Credit Exposure
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings charged off	0	5,000
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Decrease in allowance for loan losses		2,000
Troubled debt restructurings charged off			100
Nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued troubled debt restructured loans	14,000	14,000	15,300
Total Residential and Consumer Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired impaired loans	79,500	79,500	108,200
Commercial Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	350,500	350,500	457,300
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	57,708	$ 57,708	$ 72,304	94,883
Number of loans in the process of foreclosure | SecurityLoan		26	33
Mortgage loans, foreclosures		$ 1,900	$ 2,900
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 89,970	$ 89,970	$ 116,566	$ 159,089
Number of loans in the process of foreclosure | SecurityLoan		11	8
Mortgage loans, foreclosures		$ 5,300	$ 6,000