Allowance for Loan Losses and Credit Quality Information - Additional Information (Detail)	3 Months Ended		6 Months Ended						12 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) SecurityLoan	Jun. 30, 2017 USD ($) loan	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs					$ 3,800,000			$ 1,400,000
Percentage of average loans annualized, charged-offs				0.17%				0.07%
Reverse mortgage, fair value	$ 21,600,000	$ 25,300,000	$ 21,600,000	$ 21,600,000	21,600,000	$ 21,600,000	$ 21,600,000	$ 25,300,000	$ 22,600,000
Performing TDRs					18,109,000			14,100,000	14,336,000
Acquired nonimpaired loans	629,797,000	330,660,000	629,797,000	629,797,000	629,797,000	629,797,000	629,797,000	330,660,000	724,100,000
Acquired impaired loans	11,700,000		11,700,000	11,700,000	11,700,000	11,700,000	11,700,000		12,800,000
Contractual principal balance	12,900,000		12,900,000	12,900,000	12,900,000	12,900,000	12,900,000		15,000,000
Acquired impaired loans					31,147,000			27,072,000	33,614,000
Interest income on impaired loans	400,000	200,000			700,000			300,000
Accrued troubled debt restructured loans	14,000,000		14,000,000	14,000,000	14,000,000	14,000,000	14,000,000		12,400,000
Troubled debt restructuring related reserves	2,100,000		$ 2,100,000	2,100,000	2,100,000	$ 2,100,000	$ 2,100,000		1,300,000
Usual sustained repayment performance period			6 months
TRD defaulted						8	2
Subsequent default, loan amount					600,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	130,986,000	58,423,000	$ 130,986,000	130,986,000	130,986,000	$ 130,986,000	$ 130,986,000	58,423,000
Acquired impaired loans					6,002,000			1,711,000	$ 6,183,000
Number of residential loans in the process of foreclosure | SecurityLoan						6			7
Total loans outstanding, residential loans					1,700,000				$ 3,600,000
TRD defaulted | SecurityLoan						0
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	80,074,000	67,484,000	80,074,000	80,074,000	80,074,000	$ 80,074,000	80,074,000	67,484,000
Acquired impaired loans					852,000			946,000	$ 860,000
Number of residential loans in the process of foreclosure | SecurityLoan						33			29
Total loans outstanding, residential loans					4,400,000				$ 3,700,000
TRD defaulted | SecurityLoan						1
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	44,902,000	$ 13,990,000	$ 44,902,000	44,902,000	44,902,000	$ 44,902,000	44,902,000	13,990,000
Acquired impaired loans					246,000			4,000	369,000
TRD defaulted | SecurityLoan						7
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)					300,000			0
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	125,000,000		$ 125,000,000	125,000,000	125,000,000	$ 125,000,000	125,000,000		150,500,000
Impairment loans, charge off period			90 days
Total Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 504,800,000		$ 504,800,000	$ 504,800,000	$ 504,800,000	$ 504,800,000	$ 504,800,000		$ 573,500,000