Allowance for Loan Losses and Credit Quality Information - Additional Information (Detail)	3 Months Ended						12 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2017 USD ($) SecurityLoan	Mar. 31, 2017 USD ($) loan	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs			$ 2,100,000			$ 300,000
Percentage of average loans annualized, charged-offs		0.19%				0.03%
Reverse mortgage, fair value	$ 22,500,000	$ 22,500,000	22,500,000	$ 22,500,000	$ 22,500,000	$ 24,700,000	$ 22,600,000
Performing TDRs			17,260,000			13,900,000	14,336,000
Acquired nonimpaired loans	675,607,000	675,607,000	675,607,000	675,607,000	675,607,000	354,694,000	724,100,000
Acquired impaired loans	10,700,000	10,700,000	10,700,000	10,700,000	10,700,000		12,800,000
Contractual principal balance	11,800,000	11,800,000	11,800,000	11,800,000	11,800,000		15,000,000
Interest income on impaired loans			300,000			200,000
Accrued troubled debt restructured loans	13,700,000	13,700,000	13,700,000	13,700,000	13,700,000		12,400,000
Total TDRs	28,038,000	28,038,000	28,038,000	28,038,000	28,038,000		22,787,000
Nonperforming TDRs			10,778,000				8,451,000
Troubled debt restructuring related reserves	$ 1,900,000	1,900,000	1,900,000	$ 1,900,000	$ 1,900,000		$ 1,300,000
Number of term loans modified in troubled debt restructurings | SecurityLoan				12
Maturity Date Extension | SecurityLoan				4
Usual sustained repayment performance period	6 months
TRD defaulted				7	3
Subsequent default, loan amount			700,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 141,933,000	141,933,000	141,933,000	$ 141,933,000	$ 141,933,000	107,380,000
Number of commercial loans | SecurityLoan				7			7
Total loans outstanding,commercial loans	2,200,000	2,200,000	2,200,000	$ 2,200,000	2,200,000		$ 3,600,000
Number of term loans modified in troubled debt restructurings | SecurityLoan				1
Maturity Date Extension | SecurityLoan				1
TRD defaulted | SecurityLoan				0
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	85,679,000	85,679,000	85,679,000	$ 85,679,000	85,679,000	73,240,000
Number of residential loans in the process of foreclosure | SecurityLoan				39			29
Total loans outstanding, residential loans			4,900,000				$ 3,700,000
Number of term loans modified in troubled debt restructurings | SecurityLoan				1
Maturity Date Extension | SecurityLoan				0
TRD defaulted | SecurityLoan				1
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 50,321,000	50,321,000	50,321,000	$ 50,321,000	50,321,000	15,803,000
Number of term loans modified in troubled debt restructurings | SecurityLoan				7
Maturity Date Extension | SecurityLoan				0
TRD defaulted | SecurityLoan				6
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	$ 136,000,000	136,000,000	136,000,000	$ 136,000,000	136,000,000		150,500,000
Impairment loans, charge off period	90 days
Total Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 539,600,000	$ 539,600,000	$ 539,600,000	$ 539,600,000	$ 539,600,000		$ 573,500,000