Loans - Investment in Loans by Portfolio Segment and Based on Impairment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	$ 20,482	$ 21,405	$ 20,443	$ 21,202
Provision (recovery) for loan losses	701	251	712	520
Charge-offs	(1,315)	(647)	(1,943)	(1,172)
Recoveries	119	323	775	782
Allowance, Ending balance	19,987	21,332	19,987	21,332
Allowance, Loans individually evaluated for impairment	1,172	1,637	1,172	1,637	$ 1,201
Allowance, Loans collectively evaluated for impairment	18,815		18,815		19,242
Period-end balances, Loans individually evaluated for impairment	21,695		21,695		20,594
Period-end balances, Loans collectively evaluated for impairment	2,239,528		2,239,528		2,170,068
Total loans	2,261,223		2,261,223		2,190,662
Commercial Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	12,547	12,808	11,875	12,542
Provision (recovery) for loan losses	422	(128)	867	5
Charge-offs	(896)	(303)	(985)	(378)
Recoveries	16	205	332	413
Allowance, Ending balance	12,089	12,582	12,089	12,582
Allowance, Loans individually evaluated for impairment	234	514	234	514	103
Allowance, Loans collectively evaluated for impairment	11,855		11,855		11,772
Period-end balances, Loans individually evaluated for impairment	4,702		4,702		2,017
Period-end balances, Loans collectively evaluated for impairment	989,345		989,345		940,914
Total loans	994,047		994,047		942,931
Residential Mortgage Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	5,458	5,867	5,923	5,860
Provision (recovery) for loan losses	154	255	(336)	342
Charge-offs	(208)	(203)	(307)	(384)
Recoveries	22	62	146	163
Allowance, Ending balance	5,426	5,981	5,426	5,981
Allowance, Loans individually evaluated for impairment	653	817	653	817	798
Allowance, Loans collectively evaluated for impairment	4,773		4,773		5,125
Period-end balances, Loans individually evaluated for impairment	11,943		11,943		13,086
Period-end balances, Loans collectively evaluated for impairment	971,592		971,592		957,604
Total loans	983,535		983,535		970,690
Consumer Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	2,477	2,730	2,645	2,800
Provision (recovery) for loan losses	125	124	181	173
Charge-offs	(211)	(141)	(651)	(410)
Recoveries	81	56	297	206
Allowance, Ending balance	2,472	2,769	2,472	2,769
Allowance, Loans individually evaluated for impairment	285	$ 306	285	$ 306	300
Allowance, Loans collectively evaluated for impairment	2,187		2,187		2,345
Period-end balances, Loans individually evaluated for impairment	5,050		5,050		5,491
Period-end balances, Loans collectively evaluated for impairment	278,591		278,591		271,550
Total loans	$ 283,641		$ 283,641		$ 277,041