Loans - Investment in Loans by Portfolio Segment and Based on Impairment (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	$ 17,687,000	$ 21,116,000
Provision for loan losses	(184,000)	33,000
Charge-offs	(712,000)	(1,164,000)
Recoveries	430,000	569,000
Allowance, Ending balance	17,221,000	20,554,000
Loans individually evaluated for impairment	3,193,000	4,340,000	3,310,000
Loans collectively evaluated for impairment	14,028,000		14,377,000
Loans individually evaluated for impairment	45,803,000	48,084,000	45,897,000
Loans collectively evaluated for impairment	1,139,017,000		1,118,986,000
Total loans	1,184,820,000		1,164,883,000
Commercial Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	5,690,000	6,984,000
Provision for loan losses	158,000	490,000
Charge-offs	(15,000)	(442,000)
Recoveries	112,000	243,000
Allowance, Ending balance	5,945,000	7,275,000
Loans individually evaluated for impairment	682,000		717,000
Loans collectively evaluated for impairment	5,263,000		4,973,000
Loans individually evaluated for impairment	14,509,000		14,845,000
Loans collectively evaluated for impairment	255,896,000		238,066,000
Total loans	270,405,000		252,911,000
Residential Mortgage Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	8,517,000	9,830,000
Provision for loan losses	(871,000)	(330,000)
Charge-offs	(166,000)	(315,000)
Recoveries	186,000	152,000
Allowance, Ending balance	7,666,000	9,337,000
Loans individually evaluated for impairment	1,707,000		1,751,000
Loans collectively evaluated for impairment	5,959,000		6,766,000
Loans individually evaluated for impairment	19,244,000		19,209,000
Loans collectively evaluated for impairment	714,436,000		712,009,000
Total loans	733,680,000		731,218,000
Consumer Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	3,480,000	4,302,000
Provision for loan losses	529,000	(127,000)
Charge-offs	(531,000)	(407,000)
Recoveries	132,000	174,000
Allowance, Ending balance	3,610,000	3,942,000
Loans individually evaluated for impairment	804,000		842,000
Loans collectively evaluated for impairment	2,806,000		2,638,000
Loans individually evaluated for impairment	12,050,000		11,843,000
Loans collectively evaluated for impairment	168,685,000		168,911,000
Total loans	$ 180,735,000		$ 180,754,000