Allowance for Loan Losses and Credit Quality Information - Additional Information (Detail)	3 Months Ended		9 Months Ended						12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) SecurityLoan	Sep. 30, 2017 USD ($) loan	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs					$ 6,500,000			$ 5,900,000
Percentage of average loans annualized, charged-offs				0.19%				0.20%
Reverse mortgage, fair value	$ 21,400,000	$ 23,100,000	$ 21,400,000	$ 21,400,000	21,400,000	$ 21,400,000	$ 21,400,000	$ 23,100,000	$ 22,600,000
Performing TDRs					14,905,000			14,200,000	14,336,000
Acquired nonimpaired loans	596,466,000	766,932,000	596,466,000	596,466,000	596,466,000	596,466,000	596,466,000	766,932,000	724,100,000
Acquired impaired loans	7,300,000		7,300,000	7,300,000	7,300,000	7,300,000	7,300,000		12,800,000
Contractual principal balance	8,300,000		8,300,000	8,300,000	8,300,000	8,300,000	8,300,000		15,000,000
Acquired impaired loans					24,582,000			38,103,000	33,614,000
Interest income on impaired loans	300,000	500,000			1,000,000			800,000
Accrued troubled debt restructured loans	14,600,000		14,600,000	14,600,000	14,600,000	14,600,000	14,600,000		12,400,000
Troubled debt restructuring related reserves	1,100,000		$ 1,100,000	1,100,000	1,100,000	$ 1,100,000	$ 1,100,000		1,300,000
Usual sustained repayment performance period			6 months
TRD defaulted						14	4
Subsequent default, loan amount					3,700,000
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	77,154,000	103,537,000	$ 77,154,000	77,154,000	77,154,000	$ 77,154,000	$ 77,154,000	103,537,000
Acquired impaired loans					788,000			899,000	$ 860,000
Number of residential loans in the process of foreclosure | SecurityLoan						32			29
Total loans outstanding, residential loans					3,300,000				$ 3,700,000
TRD defaulted | SecurityLoan						3
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	120,987,000	$ 175,570,000	$ 120,987,000	120,987,000	120,987,000	$ 120,987,000	120,987,000	175,570,000
Acquired impaired loans					5,235,000			9,691,000	$ 6,183,000
Number of residential loans in the process of foreclosure | SecurityLoan						3			7
Total loans outstanding, residential loans					200,000				$ 3,600,000
TRD defaulted | SecurityLoan						0
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for impairment loans			90 days
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in allowance for loan losses (less than)					0			80,000
Troubled debt restructurings charged off (less than)					0			$ 100,000
Total Residential and Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	117,300,000		$ 117,300,000	117,300,000	117,300,000	$ 117,300,000	117,300,000		150,500,000
Impairment loans, charge off period			90 days
Total Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 479,200,000		$ 479,200,000	$ 479,200,000	$ 479,200,000	$ 479,200,000	$ 479,200,000		$ 573,500,000