Loans - Investment in Loans by Portfolio Segment and Based on Impairment (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance				$ 19,087				$ 17,712	$ 19,087	$ 17,712	$ 17,687
Provision (recovery) for loan losses	$ 1,215	$ 721	$ 842	1,475	$ 1,493	$ 1,344	$ 395	2,155	4,253	5,387	2,135
Charge-offs									(3,598)	(5,634)	(3,826)
Recoveries									1,460	1,622	1,716
Allowance, Ending balance	21,202				19,087				21,202	19,087	17,712
Allowance, Loans individually evaluated for impairment	2,059				3,016				2,059	3,016
Allowance, Loans collectively evaluated for impairment	19,088				16,071				19,088	16,071
Period-end balances, Loans individually evaluated for impairment	26,250				31,548				26,250	31,548
Period-end balances, Loans collectively evaluated for impairment	1,988,697				1,487,929				1,988,697	1,487,929
Total loans	2,016,141				1,519,477				2,016,141	1,519,477
Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Loans acquired with deteriorated credit quality	55								55
Period-end balances, Loans acquired with deteriorated credit quality	1,194								1,194
Commercial Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance				10,824				8,077	10,824	8,077	5,690
Provision (recovery) for loan losses									2,590	5,611	2,922
Charge-offs									(1,565)	(3,722)	(1,268)
Recoveries									693	858	733
Allowance, Ending balance	12,542				10,824				12,542	10,824	8,077
Allowance, Loans individually evaluated for impairment	516				1,271				516	1,271
Allowance, Loans collectively evaluated for impairment	11,971				9,553				11,971	9,553
Period-end balances, Loans individually evaluated for impairment	3,356				6,018				3,356	6,018
Period-end balances, Loans collectively evaluated for impairment	818,066				498,240				818,066	498,240
Total loans	822,616				504,258				822,616	504,258
Commercial Loans [Member] | Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Loans acquired with deteriorated credit quality	55								55
Period-end balances, Loans acquired with deteriorated credit quality	1,194								1,194
Residential Mortgage Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance				5,538				6,630	5,538	6,630	8,517
Provision (recovery) for loan losses									1,305	(464)	(974)
Charge-offs									(1,218)	(761)	(1,301)
Recoveries									235	133	388
Allowance, Ending balance	5,860				5,538				5,860	5,538	6,630
Allowance, Loans individually evaluated for impairment	1,145				1,245				1,145	1,245
Allowance, Loans collectively evaluated for impairment	4,715				4,293				4,715	4,293
Period-end balances, Loans individually evaluated for impairment	16,140				17,485				16,140	17,485
Period-end balances, Loans collectively evaluated for impairment	903,891				781,136				903,891	781,136
Total loans	920,031				798,621				920,031	798,621
Consumer Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance				$ 2,725				$ 3,005	2,725	3,005	3,480
Provision (recovery) for loan losses									358	240	187
Charge-offs									(815)	(1,151)	(1,257)
Recoveries									532	631	595
Allowance, Ending balance	2,800				2,725				2,800	2,725	$ 3,005
Allowance, Loans individually evaluated for impairment	398				500				398	500
Allowance, Loans collectively evaluated for impairment	2,402				2,225				2,402	2,225
Period-end balances, Loans individually evaluated for impairment	6,754				8,045				6,754	8,045
Period-end balances, Loans collectively evaluated for impairment	266,740				208,553				266,740	208,553
Total loans	$ 273,494				$ 216,598				$ 273,494	$ 216,598